TWO LAND SNAIL SPECIES OF THE MEKONG DELTA LIMESTONE HILLS (CAMBODIA, VIETNAM): AULACOSPIRA FURTIVA (EUPULMONATA: VERTIGINIDAE) AND GEORISSA CARINATA (NERITIMORPHA: HYDROCENIDAE)

We describe Aulacospira furtiva sp. n. (Pulmonata: Vertiginidae), and we expand on the protologue of Georissa carinata Sutcharit et Jirapatrasilp, 2020 (Neritomorpha: Hydrocenidae). Both species occur on limestone hills on the west flank of the Mekong delta, in Vietnam and Cambodia, and presumably are endemic to these hills. key wordS: land snails; Aulacospira; Georissa; Cambodia; Vietnam; Mekong Delta limestone hills


INTRODUCTION
The Mekong Delta Limestones (MDL) are a string of limestone hills in Vietnam and Cambodia, on the west flank of the Mekong river delta (see Vermeulen et al. 2019a). The hills are hundreds of kilometres distant from other limestone hills, which has led to high levels of endemism (Vermeulen et al. 2007(Vermeulen et al. , 2019a. In this paper we present two species. The first, Aulacospira furtiva sp. n., is presumably a site endemic species. The other, Georissa carinata Sutcharit et Jirapatrasilp, 2020, occurs widely in the MDL but so far has not been reported elsewhere.

MATERIAL AND METHODS
The descriptions and images are based on cleaned and air-dried shells. The field photographs were taken with a Sigma 105 mm macro lens mounted on a Nikon D3200 camera, with Nikon R1C1 lighting system. The collected specimens of Aulacospira furtiva were photographed with the same camera mounted on a trinocular microscope, using Helicon stacking software. The SEM images were taken with a Scanning Electron Microscope LEO 1450 VP at the Microscopic Center, Faculty of Science, Burapha University, Thailand.
The material studied derives from the private collections of the authors ('V' for coll. Vermeulen, 'A' for coll. Aiken in the lists of examined material, below). Type specimens are stored in NHMUK (Natural History Museum of the United Kingdom, London). We also cite material from CUMZ (Chulalongkorn University Museum of Zoology, Bangkok, Thailand). Duplicate sets of the cited material will be stored in institutes in Vietnam and Cambodia.
The toponymy of the area is still unsettled. We have used names of hills as in Vermeulen et al. (2019a: Vermeulen, Phung et Truong, 2007 (also from the MDL) by the presence of apertural teeth. Recently, several species of Aulacospira have been described from continental Asia with more than two apertural teeth. Most similar to A. furtiva is A. panhai Dumrongrojwattana, 2008 (east Thailand), but this species has a more depressed spire. Aulacospira lampangensis Panha et Burch, 2002 (southeast Thailand) also has a more depressed spire, as well as a distinctly shouldered body whorl;

Cross diagnosis. Differs from A. conica
Figs 1-5. Aulacospira furtiva sp. n., paratype: 1 -frontal view, shell height 3.6 mm, 2 -back view, 3 -umbilical view, 4 -apical view, 5 -apertural view (a -parietalis, b -suprapalatalis, c -palatalis, d -infrapalatalis, e -basalis, f -columellaris)  (Hidalgo, 1890(Hidalgo, [1890(Hidalgo, -1891), which is somewhat similar to A. furtiva but differs by the obtusely keeled penultimate whorl, as well as the attached last portion of the body whorl. Description. S h e l l (Figs 1-5) very small, thin, opaque, dark red-brown. Spire somewhat depressed-conical with flat or slightly convex sides; apex not protruding, rounded. Surface slightly shiny. Whorls convex; protoconch and first teleoconch whorl rounded; other whorls obtusely angular at the periphery but shouldered close to the aperture and approx. flat above the periphery, body whorl slightly rounded below, edge of umbilical impression obtusely angular, surface inside shallowly furrowed; last part of the body whorl detached. Suture deeply impressed. S c u l p t u r e . Protoconch with very fine, inconspicuous, well-spaced spiral threads. Teleoconch with unevenly spaced, somewhat raised growth lines, some developing into inconspicuous, low, obtuse riblets. Spiral sculpture: very fine, rather densely and somewhat unevenly spaced, thin, somewhat flattened threads.
A p e r t u r e (Fig. 5) free, with the parietal edge distant from the previous whorl, slightly tilted downwards with regard to the coiling axis, approx. circular to obtusely rectangular; free portion of the spire slightly narrowed towards the aperture, then expanding; teeth 5-6, all short and starting rather close to the peristome: one distinct, high, parietal lamella with an obtusely deltoid profile (marked 'a' in Fig. 5), one distinctly smaller and more rounded suprapalatalis ('b'), one infrapalatalis slightly larger than the suprapalatalis ('d'), with or without one tiny, knobshaped tooth in between these palatales ('c'), one minute, short basalis ('e'), one almost knob-shaped columellaris ('f '). Peristome gradually spreading, thin. U m b i l i c u s (Fig. 3) open, wide, with less than two whorls visible inside. D i m e n s i o n s . Height of shell 2.4-3.1 mm; width 3.2-3.6 mm; height/width 0.69-0.91; umbilicus 0.9-1.0 mm wide, which is 28-29% of the shell width; number of whorls 4-4 1/4; aperture height 1.2-1.4 mm; aperture width 1.5-1.6 mm.

Distribution.
Cambodia, Kampot Province, Kampong Trach area, Phnom Kampong Trach. Presumably a site endemic species. So far known from a single well-drained cave 22-25 m above the level of the surrounding plain. Etymology. Furtivus (Latin) -stealthy, hidden, referring to the cave where the species was found.