NEW SPECIES OF LAND SNAILS (MOLLUSCA: GASTROPODA: CAENOGASTROPODA AND PULMONATA) OF THE MEKONG DELTA LIMESTONE HILLS (CAMBODIA, VIETNAM)

Twenty-two new species of land snails are described from limestone hills on the west flank of the Mekong river delta, in Cambodia and Vietnam. The species belong to the families Assimineidae, Cyclophoridae, Diplommatinidae, Ariophantidae, Helicarionidae, Streptaxidae and Vertiginidae.


INTRODUCTION
The composition and special features of the snail fauna of isolated limestone hills on the west flank of the Mekong Delta (Cambodia, Vietnam) remained virtually unknown (except for a few species described by Van benTHem JuTTing (1962)) until the rampant quarrying of the hills raised concerns among the conservation-minded.The first biodiversity surveys in the area were conducted by the Institute of Tropical Biology, Ho Chi Minh City, Vietnam, in the last years of the 20th century.Well over a decade later, the exploration of Cambodian part of the hill chain started, spurred once more by concerns over industrial impact on the local biodiversity.By now, the surveys have yielded an extensive collection of terrestrial Mollusca.Taxonomical unravelling is ongoing; this paper is one in a series on the snail fauna of the above-mentioned limestone hills, and is preceded by Vermeulen et al. (2007), and Vermeulen et al. (in press).

THE MEKONG DELTA LIMESTONES
The Mekong Delta Limestones (MDL) consist of a string of small and medium-sized limestone hills along the Vietnamese and Cambodian coast, where the border between the two countries meets the sea (Fig. 1).The hills are situated along the western flank of the delta of the Mekong River and, although the hills themselves apparently reached sufficiently high above their surroundings to have a history as a permanently terrestrial environment, the surrounding alluvial plains have known extended periods of inundation.For land snails and other terrestrial fauna preferring limestone environments, the limestone outcrops are islands of suitable habitat in vast plains of uninhabitable territory.This isolation, combined with the fact that the limestone hills nearest to the MDL are hundreds of kilometres away has left an imprint on the snail fauna of the hills.It has been characterised as an island fauna (Vermeulen et al. 2007), with relatively few species but high rates of endemism.So far, the species list includes 113 species, of which 62 (55%) are potentially endemic to the MDL.Investigation into this fauna is ongoing and far from complete; it is likely that more species, provisionally identified as known and widespread ones, will prove new to science and endemic.
The relevance of quickly describing these new species lies in the fact that the MDL as a whole are under serious threat.Next to a severe general environmental degradation which started during the second half of the last century, four large and several smaller cement factories are active in the area, which, together with numerous local entrepreneurs, quarry the valuable limestone.Fig. 1 shows the state of affairs in 2016, with extant limestone hills (grey) and extinct ones (light green).Conservation organisations involved in nature conservation wish to make visible the degradation of the biodiversity, making use of the categories of the IUCN Red List Program (IUCN red lisT https://www.iucnredlist.org/)to describe the increasing pressure on individual species.For inclusion in the IUCN Red List, species need to be formally described.Distribution maps of the species are presented to demonstrate not only the wealth of the local fauna, but particularly the fact that many species appear to have a very limited distribution, even within the MDL.These distribution patterns may help NGO's to prioritize their efforts.

MATERIAL AND METHODS
The material studied derives from the private collection of the first author ('V' in the lists of exam-ined material, below).Type specimens of new species are stored in NHMUK (Natural History Museum of the United Kingdom, London) and RMNH (National Museum of Natural History, formerly Rijksmuseum van Natuurlijke Historie, the Netherlands, Leiden).Duplicate sets of the collection will be stored in institutes in Vietnam and Cambodia.The illustrations were drawn by the first author, with the aid of a Wild M8 stereo microscope with a Camera Lucida device.
The first author claims copyright of the illustrations of the snail species.
The toponymy of the study area appears somewhat unsettled.We have used names of hills originating from various sources, some less formal than others.Therefore, we add a table with the hill names used in this paper, with coordinates (Table 1).

Cross diagnosis.
Uniquely characterised within the genus by the radial ribs developing into semi-elliptic lamellae above the periphery.Description.Shell minute, rather thin, somewhat translucent, white.Surface shiny.Spire conical with slightly convex sides, apex obtuse, whorls convex, the last one slightly shouldered.Sculpture: protoconch minutely punctate, first part of teleoconch (up to 1 1/2-1 3/4 whorl) minutely punctate as the protoconch; other whorls with prosocline radial ribs, those on the last whorl below the periphery rather low and flat, densely placed but somewhat unevenly spaced, distinctly sinuous somewhat below the periphery approx.at the level of the suture of the penultimate whorl, most radial ribs rather abruptly ending approx. at the level of the periphery, but each at a slightly different level, and some with a slightly thickened and higher terminal part; one out of two to four radial ribs continuing above the periphery, where they develop into widely and somewhat unevenly spaced, thick, semi-elliptic lamellae with a rounded edge, which rather abruptly end well below the suture.Spiral sculpture absent.Aperture approx.obliquely and widely inverted-ovate in outline, with a straight parietal side, transition from parietal to basal side narrowly rounded to obtusely angular.Fresh-looking specimens have been found in cave clay deposits as well as in organic soil deposits at the bottom of karren crevices and in the twilight-zone in caves.
Distribution.Mekong Delta Limestones, but probably not endemic.We have unidentified material that could be included in the genus from elsewhere in the region.
Etymology: spelaion (Greek) -cave, cyclotus -referring to the genus Cyclotus Speleocyclotus holcobasis sp.n. the last half-whorl widened.Whorls convex, rounded, periphery of the last whorl slightly narrower rounded or not, suture impressed.Sculpture: protoconch punctate; teleoconch: first whorls with evenly spaced, thin, low radial riblets, the first ones rather closely placed, then somewhat more widely spaced, with 14-22 ribs on the first 1/2 whorl of the teleoconch, surface in between riblets smooth, locally with traces of spiral striation; outer whorl with densely placed, somewhat unevenly spaced radial riblets with thin, rather low calcareous crests which rather easily erode, and are most persistent towards the suture and around the umbilicus, with a slightly more prominent radial riblet per 4-10 riblets (at approx.the same interval as the most widely spaced riblets on the first whorls, most conspicuous on the lower surface of the shell); towards the aperture radial riblets more prominent, with more persistent calcareous crests; outer whorl without spiral striation.Aperture approx.circular.Peristome slightly thickened and widened.Dimensions: height 1.7-2.0mm; width 3.0-3.8mm; h/w 0.46-0.63;diameters of the first three whorls 0.5-0.6 mm, 1.2-1.3mm, 2.4-3.1 mm respectively; number of whorls 3 1/8-3 5/8; umbilicus 0.8-1.2mm wide, which is 0.26-0.34 of the shell width; height aperture 1.0-1.3mm; width 1.2-1.6 mm.Ecology.In deep soil deposits in (moderately disturbed) partly deciduous woodland on limestone bedrock; in the twilight zone of caves.Distribution.Cambodia, Kampot Province, Phnom La'Ang, including two satellite hills.Presumably a site endemic species.
Etymology: the name refers to the type locality.

Cross diagnosis.
Identified within the genus by the mode of coiling, with the first three whorls significantly wider than in the other species, and the much less conspicuous sculpture.Description.Shell (very) small, dextral, white.Spire low-conical with the last half-whorl widened.Whorls convex, rounded, suture impressed.Sculpture: protoconch punctate; first whorls of teleoconch with evenly spaced, thin, low radial riblets, the first ones rather closely placed, then widely spaced, with 20-22 ribs on the first 1/2 whorl of the teleoconch, surface in between riblets smooth, without spiral striation; outer whorl with somewhat unevenly spaced growth-lines, often with slightly more prominent growth-lines at approx. the same interval as the most widely spaced riblets on the first whorls; towards the aperture the growth-lines are more prominent, some developing in radial riblets without calcareous crests, sometimes with an obsolete peristome in between; outer whorl without spiral striation.Aperture somewhat elliptic.Peristome slightly thickened and widened.Dimensions: height 2.4-2.7 mm; width 4.6-5.2mm; h/w 0.52-0.58;diameters of the first three whorls 1.0-1.1 mm, 2.0-2.3 mm, 4.7-5.0mm respectively; number of whorls 2 3/4-3 1/4; umbilicus 1.0-1.3mm wide, which is 0.22-0.28 of the shell width; height aperture 1.8-2.0mm; width 2.0-2.Diplommatina decapitata sp.n.Description.Shell very small, dextral, decollate, fusiform with the penultimate whorl widest.Spire conical with approx.flat sides.Whorls convex.Suture impressed.Constriction level with the angular edge of the peristome or slightly beyond this point, up the spire, with 2 lamellae: 1 low and thin parietalis which continues approx.1/2 whorl into the tuba, 1 distinct, high columellaris which continues to the aperture.Sculpture: radial ribs slightly prosocline, straight but somewhat curved above the aperture, on the tuba, rather low and thin, rather densely placed on the penultimate whorl (5-7 ribs/0.5 mm), more widely spaced on the previous whorl (3-4 ribs/0.5 mm); spiral striation present, inconspicuous or lo-cally absent in some shells.Aperture hardly tilted with regard to the coiling axis; columellaris distinct, downwards directed.Peristome double, expanding; palatal side slightly sinuous, rounded; basal side broadly rounded; basal edge not sinuous, narrowly rounded; outer peristome expanding beyond the inner, widely so on the palatal side and in the basal edge; inner peristome slightly protruding from the outer, with a palatal lip, not expanding over the shell on the columellar side, little expanding on the parietal side.Umbilicus closed.Dimensions: height 2.0-2.3 mm (decollated shell); width 1.0-1.15mm; h/w 1.8-2.2;number of whorls 5 1/2-6 1/2, including a tuba of 1-1 1/8 whorl; height aperture 0.4-0.5 mm; width 0.4-0.45mm.

Cross diagnosis. Identified among Continental
Asiatic Plectostoma with the aperture to the left of the spire by the sinuous peristome, with the upper right side conspicuously protruding and the upper left side receding.In strictly frontal view this is not so obvious; in left lateral view the structure is more easily visible.Plectostoma tonkinianum (Dautzenberg et Fischer, 1905), from the MDL, and illustrated in liew et al. (2014: 62), also differs by having a more distinctly ovoid spire, with the last whorl much more narrowly coiled than the penultimate.Description.Shell very small, dextral with a sinistral tuba.Spire conical with convex sides, almost ovoid.Apex slightly oblique.Whorls convex, rounded, suture impressed.Constriction with 4 teeth: 1 parietalis a little into the tuba; 1 longitudinal palatalis close to the suture, 1 transverse palatalis; 1 small, knob-shaped columellaris.Tuba coiled around an axis different from the axis of the spire, gradually narrowed towards the constriction, approx.rounded below.Sculpture: radial ribs on the spire prosocline, slightly sinuous or with a shallow loop to the right, but those towards the tuba not sinuous; rather closely placed (8-9 ribs/0.5 mm on the last whorl of the spire; 7-9 ribs/0.5 mm on the penultimate whorl); those on the tuba not sinuous; a fine spiral striation present.Aperture not or hardly tilted with regard to the coiling axis, positioned to the left of the spire, level with approx.the start of its penultimate whorl, approx.obtusely rectangular, with the upper right side conspicuously protruding, and the upper left side receding.Peristome touching the spire, simple, on the outside without congested lamellae, or with a few very thin ones; slightly spreading, thickened, with or without a slight lip along the palatal side.Dimensions: height of spire without tuba 2.8-3.3 mm; width 1.8-2.0mm; h/w 1.5-1.9;width including tuba 3.0-3.2mm; umbilicus 0.2-0.3mm wide; number of whorls 5 7/8-6 3/8, excluding the tuba of c. 1/2 whorl; height aperture (measured on the inside) 0.8-0.9mm; width 0.9-1.0mm.

Family Ariophantidae Godwin-Austen, 1888
Macrochlamys psyche sp.n.Note.We provisionally include this in Macrochlamys, using the genus as a receptacle of medium-sized, approx.lenticular species of unknown affinities.Etymology: psyche (Greek) -ghost, a noun in apposition, referring to the pallid appearance of the species in combination with the eternal darkness of its habitat.

Microcystina bataiensis sp. n.
Figs  with similar sculpture and a closed umbilicus, differs by the triangular spur protruding from the columellar peristome and covering the umbilicus.In addition, the shell of M. exul is more loosely coiled than any of the species mentioned above, compare the diameters of the third whorl.
Description.Shell very small, rather thin, hardly translucent, white.Outline lenticular; spire little elevated, narrowly rounded.Surface with a silky lustre.Protoconch whorls somewhat convex, next whorls somewhat convex, last whorl somewhat narrowly rounded at the periphery, above and below the periphery broadly rounded, suture not or hardly impressed, umbilical area shallowly impressed.Sculpture: protoconch approx.smooth at 40× magnification; transition to teleoconch hardly marked; teleoconch above and below the periphery with growth-lines at irregular intervals developing into distinct, coarse, rather wide but shallow, distinctly curved radial grooves which slightly curve in the direction of shell growth towards the suture, and which are less conspicuous on the first 1 3/4-2 1/4 whorls; spiral sculpture subordinate to the radial sculpture, rather inconspicuous, fine, densely placed, shallow grooves above and below the periphery.Aperture obliquely crescent-shaped, somewhat obtusely angular at the periphery, broadly rounded above and below.Peristome not expanding, in fully adult specimens slightly thickened on the palatal and basal side.Umbilicus closed.Dimensions: height 1.2-2.0mm; width 2.6-4.1 mm; h/w 0.41-0.57;diameters of the first four whorls 0.7-0.9mm, 1.4-1.6 mm, 2.5-2.8mm, 3.8-4.0mm respectively; number of whorls 3 7/8-4 1/2; height aperture 1.3-1.5 mm; width aperture 2.0-2.5 mm.Ecology.Soil deposits in disturbed semi-deciduous and secondary (coastal) woodland on limestone bedrock, soil deposits in the twilight zone of caves.Distribution.Vietnam, Kien Giang Province, Kien Luong District, Nui Chua Hang.Presumably a site endemic species.Etymology: exul (Latin), a noun in apposition, a banned person, referring to its occurrence at the far end of the MDL.
Microcystina obliquestriata sp.n.Cross diagnosis.Shares the radial sculpture and closed umbilicus with M. exul, above; differs by having a more densely coiled shell.In addition, the radial sculpture curves away from the direction of shell growth when approaching the suture (curving towards the direction of the shell growth in M. exul).
Description.Shell very small, rather thin, hardly translucent, white.Outline lenticular; spire usually somewhat elevated, rounded.Surface with a silky lustre.Protoconch whorls somewhat convex, next whorls somewhat convex, last whorl somewhat narrowly rounded at the periphery, above and below the periphery broadly rounded, suture not or hardly impressed, umbilical area shallowly impressed.Sculpture: protoconch approx.smooth at 40× magnification; transition to teleoconch hardly marked; teleoconch above and below the periphery with growth-lines, on most of the shell surface developed into rather inconspicuous, very fine, densely placed radial riblets, in addition to these with distinct, coarse, usually widely but unevenly spaced, but in some shells locally rather densely placed, rather wide but shallow, distinctly curved radial grooves which curve away from the direction of shell growth towards the suture; spiral sculpture subordinate to all radial sculpture, locally traces of inconspicuous, fine, rather densely placed, shallow grooves above and below the periphery.Aperture obliquely crescent-shaped, somewhat obtusely angular at the periphery, broadly rounded above and below.
Peristome not expanding, somewhat thickened on the palatal and basal side.Umbilicus closed.Dimensions: height 1.5-1.9mm; width 3.0-3.7 mm; h/w 0.48-0.56;diameters of the first four whorls 0.6-0.7 mm, 1.1-1.3mm, 1.8-2.2mm, 2.6-3.1 mm respectively; number of whorls 4-4 5/8; height aperture 0.9-1.'Kien Luong': Endangered B1ab(i,ii,iii,iv,v) + 2ab(i,ii,iii,iv,v).Re-assessment is necessary because of new distribution data.Among MDL species, it stands out by its sculpture, which gives the shell surface a silky sheen, as in the species described above, but it lacks the wide, shallow radial grooves present in those species.In addition, the shell is less flattened.Description.Shell very small, thin, somewhat translucent, white.Outline lenticular; spire somewhat elevated, narrowly rounded.Surface with a silky lustre.Protoconch whorls somewhat convex, next whorls somewhat convex, last whorl rounded at the periphery and above, below the periphery broadly rounded, suture not or hardly impressed, umbilical area rather shallowly impressed.Sculpture: protoconch approx.smooth at 40× magnification except for a very fine, dense spiral striation, transition to teleoconch hardly marked; teleoconch above the periphery growthlines, on parts of the shell surface developed into rather inconspicuous, unevenly spaced, fine densely placed radial riblets, which end approx.perpendicular to the suture; spiral sculpture subordinate to all radial sculpture, inconspicuous, very fine (just visible at 40× magnification), very densely placed, shallow grooves; below the periphery with growth-lines only, and some traces of spiral striation.Aperture obliquely crescent-shaped, rounded at and above the periphery, broadly rounded below.Peristome not expanding, not or hardly thickened on the palatal and basal side.Umbilicus open, sometimes approx.closed.Dimensions: height 1.9-2.3mm; width 3.4-3.9mm; h/w 0.54-0.61;diameters of the first four whorls 0.65-0.80mm, 1.1-1.5 mm, 1.9-2.3mm, 3.1-3.7 mm respectively; number of whorls 4-4 1/2; umbilicus up to 0.15 mm diameter, which is up to 0.06 of the shell diameter; height aperture 1.3-1.5 mm; width aperture 1.7-2.2mm.Ecology.Soil deposits in disturbed semi-deciduous woodland and secondary woodland on limestone bedrock, soil deposits in the twilight zone of caves, also in the dark zone of caves, around Ficus-roots.iucn red lisT Category, as Sesara sp.nov.'Bai Voi': Critically Endangered B1ab(i,ii,iii,v) + 2ab(i,ii,v).New distribution data make necessary re-assessment.
Examined material.Cambodia: Kampot Province, Banteay Meas area: Phnom Koun Sat (V 15867/6 shells); Phnom Teuk Srok, north hill (V 15865/5); ditto, south hill (holotype: NHMUK 20180569, paratype: V 15866/1).Vietnam: Kien Giang Province, Kien Luong District: Nui Bai Voi, southwest flank, rocky, locally steep limestone slope (V 14689/3).Cross diagnosis.Similar to S. bouyei (Crosse, 1863) (Vietnam, 'Pulau Condor' = Con Son Island) but larger (a probable type specimen of S. bouyei, measured from a photograph: height c. 3.2 mm; width c. 6.2 mm; diameters of the first four whorls c. 0.8 mm, c. 1.5 mm, c. 2.6 mm, c. 4.2 mm respectively; number of whorls c. 5 1/8).In addition, Sesara bouyei has a white shell, and a shorter palato-basal lamella.Sesara triodon Tanmuangpak, Tumpeesuwan et Tumpeesuwan, 2017 (Thailand), also has a lenticular, smooth shell with a rounded periphery.This species is characterised by the deeply concave palato-basal side of the aperture, and an additional supra-palatal knob on the peristome.Description.Shell rather small, thin, slightly translucent, somewhat greenish corneous.Outline lenticular; spire little elevated, rounded.Surface glossy.Protoconch whorls almost flat, next whorls slightly convex, last whorl well-rounded at the periphery, but towards the aperture obtusely angular at the periphery.Sculpture: protoconch smooth or slightly and minutely pitted (just visible at 40× magnification); teleoconch with a few inconspicuous growth-lines; spiral sculpture inconspicuous, locally with very fine, densely placed but somewhat unevenly spaced, shallow grooves above as well as below the periphery.Aperture obliquely crescent-shaped, obtusely angular at the periphery, broadly rounded on the basal side, approx.flat or slightly concave in between.Peristome not expanding, thickened on the palatal and basal side, and with 2(-3) teeth: 1 inconspicuous to distinct, knob-shaped, rounded to obtuse basalis; 1 distinct, transverse palato-basal lamella which is usually (rather) abruptly cut off at both ends, and has a slightly convex to deeply concave, sometimes unevenly serrated crest, so that it seems almost divided into 2 separate teeth.Umbilicus closed.Dimensions: height 4.8-6.0mm; width 9.5-12.0mm; h/w 0.50-0.55;diameters of the first four whorls 1.0-1.3mm, 1.8-2.3mm, 3.1-4.0mm, 5.1-6.5 mm respectively; number of whorls 5 1/2-6 1/8; height aperture 3.3-4.0mm; width aperture 5.0-6.0 mm.Ecology.In disturbed semi-deciduous woodland and secondary woodland on limestone bedrock.Distribution.Cambodia, Kampot Province, Banteay Meas area.Vietnam, Kien Giang Province, Kien Luong District.Presumably endemic to the MDL.Etymology: politus (Latin) -polished Thailand), S. sesarella is distinguished by the shallow, wide radial furrows on the middle whorls, as well as by the presence of a parietal lamella.In addition, it is distinctly smaller.Description.Shell very small, rather thin, translucent, white to pale corneous.Outline approx.lenticular to almost discoid; spire little elevated, rounded.Surface shiny.Protoconch whorls almost flat, next whorls slightly convex, last whorl well-rounded at the periphery, broadly rounded below.Sculpture: protoconch with a few traces of radial folds, with very fine, inconspicuous, densely placed, very shallow, vaguely delineated spiral grooves; first whorl of teleoconch with some inconspicuous growth-lines, from approx. the start of the second whorl to approx.the start of the last whorl with widely and more or less evenly spaced, distinct, shallow and wide radial furrows, last whorl with a few scattered, unevenly spaced, inconspicuous, shallow, narrow furrows which above the periphery, and some scattered growth-lines below the periphery; spiral sculpture as on the protoconch, present above as well as below the periphery.Aperture obliquely crescent-shaped, palatal and basal side evenly rounded.Peristome not expanding, thickened on the palatal and basal side, and with 3 teeth: 1 thick, transverse, rounded parietal lamella which is highest towards the periphery and rather abruptly stops well before reaching the angular corner; 1 more or less transverse, low, knob-shaped, rounded palato-basalis; 1 small, knob-shaped palatalis.Umbilicus closed (open and narrow in juveniles).Dimensions: height 1.35-1.50mm; width 2.7-2.9 mm; h/w 0.48-0.52;diameters of the first four whorls 0.6-0.7 mm, 1.2-1.3mm, 1.8-2.0mm, 2.6-2.8mm respectively; number of whorls 4-4 1/4; height aperture 0.9-1.0mm; width aperture 1.4-1.5 mm.Note.We list all MDL Vertiginidae, with the material examined, for the distribution maps, see Fig. 52 (the soil dwellers with white shells: Acinolaemus), and Fig. 53 (rock dwellers with brown shells: Anauchen, Aulacospira, and Hypselostoma).

Acinolaemus carcharodon Vermeulen, Phung et Truong, 2007
Fig. 52 (orange dots).Vermeulen et al. 2007: 87.iucn red lisT Category: Vulnerable B1ab(i,ii,iii,iv,v) + 2ab (i,ii,iii,iv,v).Description.Shell very small, thin, opaque, dark red-brown with some whitish growth-lines.Spire (high-)conical with concave sides; apex somewhat protruding, rounded.Surface with a silky lustre.Whorls: protoconch and first teleoconch whorls convex, penultimate whorl moderately convex, last whorl narrowly rounded at the periphery, slightly convex above, somewhat shouldered in the last 1/2 whorl, almost flat below the periphery, edge of umbilical impression obtusely angular, surface inside somewhat concave with a deep furrow just below the edge; last part of the last whorl detached, with the aperture distant from the spire, obliquely downwards directed, slightly to distinctly so.Suture deeply impressed.Sculpture: protoconch somewhat granular, with very fine, inconspicuous, well-spaced spiral threads; teleoconch with some scattered, unevenly spaced, flat growth-lines, in some shells locally with inconspicuous, low, obtuse, unevenly spaced riblets in between; spiral sculpture predominant, very fine, widely but somewhat unevenly spaced, thin, somewhat flattened threads.Aperture free, with the parietal edge widely distant from the previous whorl, slightly tilted downwards with regard to the coiling axis, approx.circular to somewhat elliptic, free portion of the spire gradually narrowed towards the aperture, then expanding; teeth 6-7: 1 distinct, high, rather long parietal lamella with a rounded profile, starting rather deep inside the aperture and somewhat abruptly ending at both ends, 1 antrorsely hook-shaped, rather small supra-palatalis deep inside, 1 similar but much larger palatalis, with or without 1 inconspicuous, small, short, low, ridgeshaped infrapalatis, 1 antrorsely hook-shaped, rather small infracolumellaris, 1 rather long, high columellar lamella,1 very small, knob-shaped infraparietalis.& burcH 2002).It also has a narrower umbilicus.
Among Anauchen from the MDL it is characterised by the presence of a peripheral ridge on the last whorl.Description.Shell very small, thin, opaque, dark red-brown with (some) whitish growth-lines, sometimes pale corneous to white.Spire (depressed-)conical with concave sides; apex somewhat protruding, rounded.Surface rather dull.Whorls: protoconch and first teleoconch whorls convex, penultimate whorl (moderately) convex, last whorl with an ob-tuse, distinctly protruding peripheral ridge, convex and distinctly shouldered above, convex below, edge of umbilical impression narrowly rounded to obtusely angular, surface inside somewhat concave and with a (deep) furrow just below the edge; last part of the last whorl somewhat detached, with the aperture close to the spire or somewhat distant from it, horizontal or slightly downwards directed.Suture deeply impressed.Sculpture: protoconch somewhat granular; teleoconch with a minutely granulate sur- face (just visible at 40× magnification), with some scattered, unevenly spaced, flat growth-lines; spiral sculpture predominant, rather fine, moderately to widely but somewhat unevenly spaced, wavy, thin, somewhat flattened threads which are shiny in contrast with the dull shell surface.Aperture free, with the parietal edge slightly to widely distant from the previous whorl, slightly tilted downwards with regard to the coiling axis or not, approx.circular to obtusely rectangular, free portion of the spire gradually narrowed towards the aperture, then abruptly expanding; teeth 4-5: 1 distinct, high, short parietal lamella with a rounded profile, starting rather deep inside the aperture and abruptly ending at both ends, 1 similarly shaped but small supra-palatalis, 1 similarly shaped, rather small palatalis with a proximally thickened crest, with or without 1 inconspicuous, small, short, low, ridge-or almost knob-shaped infrapalatis, with 1 rather low, rather short columellar lamella.Peristome rather abruptly spreading, thin, with a distinct convexity on the upper palatal side, close to the edge, corresponding with an indentation on the outside.Umbilicus open, wide, with less than two whorls visible inside.Dimensions: height of shell 2.6-3.5 mm; width 3.0-3.7 mm; height of spire (excluding free portion of the last whorl) 1.9-2.6 mm; width 2.0-2.4 mm; h/w 0.90-1.15;umbilicus 0.7-1.0mm wide, which is 33-45% of the spire width; number of whorls 4 1/8-5 1/8; height aperture 1.    Description.Shell very small, thin, opaque, pale brown-corneous with some whitish growth-lines, to white.Spire high-conical with concave sides; apex somewhat protruding, rounded.Surface slightly shiny.Whorls: protoconch and first teleoconch whorls convex, penultimate whorl moderately convex, last whorl with an obtuse, distinctly protruding peripheral ridge, above this slightly convex to slightly concave, immediately below the periphery deeply concave and furrowed, lower down flat to slightly convex all the way down to the edge of the umbilical impression, edge of umbilical impression obtusely angular, surface inside about flat and with a furrow just below the edge, last part of the last whorl detached,

Fig. 1 .
Fig. 1.The Mekong Delta Limestones (MDL).Underlying map: red -MDL outcrops in four groups A, B, C, and D. Inset maps A, B, C, and D: grey -extant, limestone outcrops, light green -extinct, limestone outcrops, no longer of significance for limestone biodiversity

Fig. 52 .
Fig. 52.Distribution of soil-dwelling Vertiginidae endemic to the MDL, genus Acinolaemus.Dots of different colours represent different species: orange -Acinolaemus carcharodon; yellow -A.pyramidalis; red -A.rectus Peristome gradually spreading, thin, with a distinct convexity on the upper palatal side, close to the edge, corresponding with an indentation on the outside.Umbilicus open, wide, with more than two whorls visible inside.Dimensions: height of shell 3.4-4.4mm; width 3.1-3.7 mm; height of spire (excluding free portion of the last whorl) 2.6-3.0 mm; width 2.1-2.2mm; h/w 1.18-1.43;umbilicus 0.7-1.0mm wide, which is 32-48% of the spire width; number of whorls 5-6; height and width aperture 1.2-1.7 mm.Ecology.Rock surfaces in disturbed to degraded, semi-deciduous woodland and secondary vegetation on limestone bedrock.Distribution.Cambodia, Kampot Province, Kampong Trach area, Phnom Kampong Trach.Presumably a site endemic species.Notes.The large lamella on the parietal side of the aperture is well-rounded in profile, without a notch or lower portion somewhere along its length that may indicate that it is a composite of an angularis and a parietalis.If this were the case, the species should be included in Hypselostoma.Etymology: chaunos (Greek) -lax, wide, salpinx (Greek) -trumpet Anauchen depressus sp.n.

Table 1 .
The list of hills of the study area

Speleocyclotus laangensis sp. n.
Shares with S. microcoryphe the absence of a periomphalic furrow and the rather dense mode of coiling.The differences between the two are given below S. microcoryphe.