Figured lectotype, ANSP 91323 (Fig. 1A); paralectotypes (from same lot), ANSP 420575.

TEXAS. USNM 123757, USNM 134007, Guadalupe River. ANSP 134247, Nueces River. Bell County: UMBMNH uncat., Salado Creek, Salado, old US 81 (30.944°N, 97.539°W), 14.IV.1972. Comal County: ANSP 90562, drift of Guadalupe River, 3.2 km above New Braunfels (29.756°N, 98.138°W). UMBMNH uncat., Spring Branch, west of Spring Branch (29.891°N, 98.435°W), 28.III.1963. USNM 473488, New Braunfels (29.702°N, 98.124°W). Kerr County: USNM 874910, South Fork Guadalupe River, ca. 56.4 km northwest of Leakey (29.957°N, 99.456°W), 30.XII.1979. USNM 874932, spring run adjacent to South Fork of Guadalupe River, ca. 11.2 km southwest of Hunt (30.005°N, 99.409°W), 30.XII.1979. USNM 883412, North Fork Guadalupe River (30.054°N, 99.486°W), 26.IV.1993. USNM 874923, North Fork of Guadalupe River at Riverbend Ranch crossing at FM 1340 (30.065°N, 99.373°W), 29.XII.1979. USNM 251887, Japonica (30.064°N, 99.344°W). Kimble County: UMBMNH uncat., Llano River at FM 385, 25.6 km northeast of Junction (30.589°N, 99.598°W), 12–13.IV.1972. Kinney County: UMBMNH uncat., Nueces River, Tularosa Road, A.G. “Tony” Rose Ranch (29.518°N, 100.271°W), 9.VI.1971. USNM 883413, West Nueces River above crossing on Tularosa Road near Spring Ranch, just below Silver Lake (29.523°N, 100.251°W), 25.IV.1993. Real County: UMBMNH uncat., USNM 874926, Old Faithful Spring outflow, Hwy 55, 0.8 km north of Camp Wood (29.680°N, 100.015°W), 8.VI.1971, 31.XII.1979. FMNH 283564, FMNH 283565, FMNH 283573, FMNH 287574, Old Faithful Spring, 1.0 km north of Camp Wood (29.680°N, 100.015°W), 27.I.2001. USNM 883414, Leakey Springs run at Hwy 337 crossing, ca. 0.48 km east of Leakey (29.723°N, 99.757°W), 25.IV.1993. FMNH 283561, Leakey Springs creek, 0.64 km east of Leakey (29.723°N, 99.757°W), 26.I.2001. Uvalde County: UMBMNH uncat., Nueces River, Chalk Bluff, 24 km northwest of Uvalde (29.359°N, 99.984°W), 27.V.1971. USNM 883477, spring at Camp Chalk Bluff, tributary to East Nueces River (29.362°N, 99.984°W), 23.IV.1993. USNM 883666, USNM 883421, East Nueces River at 19 Mile Crossing, 1.6–3.2 km south of Hwy 55 and FM 334 (29.398°N, 100.00°W), 31.XII.1979, 29.IV.1993. UMBMNH uncat., USNM 883420, East Nueces River, Hwy 55, Lake Nueces County Park (29.619°N, 100.01°W), 1-VI-1971, 29.IV.1993.

Shell large for genus (maximum height, 4.6 mm), ovate-conic, openly umbilicate; penis with short filament and oblique, squarish lobe bearing a single terminal gland along its distal edge. Distinguished from congeners having closely similar shells and penes as follows: from Marstonia gaddisorum Thompson by its less convex shell whorls, distinctive pallial roof pigmentation, larger number of cusps on the inner side of the lateral teeth and on the outer marginal teeth, larger penial lobe, narrower terminal gland, and smaller overlap of the bursa copulatrix by the albumen gland; from Marstonia lustrica by its smaller prostate gland, smaller penial lobe, narrower penial filament, straight anterior vas deferens, partly imbedded (in albumen gland) bursal duct, and larger seminal receptacle; and from Marstonia ogmorhaphe Thompson by its smaller size, broader shell, smaller prostate gland, straight anterior vas deferens, and smaller bursa copulatrix.

Shell ovate-conic, (Fig. 1A–C); height about 2.6–4.6 mm; whorls 4.5–5.5. Protoconch near planispiral, slightly tilted (Fig. 1D), initial 0.75–1.0 whorl strongly wrinkled (Fig. 1E). Teleoconch whorls weakly convex, often narrowly shouldered, rarely having subsutural angulation; sculpture of strong collabral growth lines, later whorls having numerous weak spiral striae. Aperture pyriform or ovate. Inner lip complete across parietal wall in larger specimens, usually narrowly adnate, rarely slightly disjunct; usually thin, sometimes slightly thickened apically; columellar shelf absent or very narrow; outer lip thin or slightly thickened, orthocline or prosocline. Umbilicus open but small. Measurements of the lectotype and a live-collected series of shells from the Nueces River basin are in Table 2.

Operculum thin, amber, narrowly ovate, multispiral with eccentric nucleus (Fig. 1F); last 0.25 whorl sometimes frilled on outer side; inner side having well developed rim near outer edge (Fig. 1G); attachment scar border sometimes weakly thickened near nucleus. Radula (Fig. 2A), having about 36 well-formed rows of teeth. Central teeth about 38 µm wide, cutting edge convex (Fig. 2B); lateral cusps 3–8; central cusp pointed or hoe-shaped, parallel-sided proximally or tapering throughout; basal cusps 1–3, small; basal tongue U- or V-shaped, about as long as lateral margins. Lateral tooth face rectangular; central cusp pointed or hoe-shaped (Fig. 2C); lateral cusps 2–5 (inner), 3–7 (outer); outer wing broad, flexed, about 140% length of cutting edge; basal tongue weakly developed. Inner (Fig. 2D) and outer (Fig 2E) marginal teeth both having 14–21 cusps and basally positioned rectangular wing. Radular count data were from USNM 874926.

Cephalic tentacles pale except for black eyespots. Snout brown; distal lips pale; foot pale. Pallial roof having black pigment bands along edges of ctenidium and dorsal edge of genital duct (Fig. 1H); visceral coil pale except for black pigment on testis. Ctenidium positioned a little in front of pericardium; ctenidial filaments 24–25 (n=5), broadly triangular, lateral surfaces ridged. Osphradium narrow, positioned slightly posterior to middle of ctenidium. Hypobranchial gland large, overlapping rectum and part of genital duct, thickened alongside kidney. Style sac longer than remaining portion of stomach, posterior stomach having small caecal appendix. Testis large (1.75 whorls), composed of compound lobes, broadly overlapping stomach anteriorly. Seminal vesicle opening near anterior edge of testis, composed of a few thickened coils, positioned along ventral side of anterior 33% of testis. Prostate gland small, pea-shaped, with about 50% of length in pallial roof. Anterior vas deferens opening from antero-ventral edge of prostate gland, section of duct on columellar muscle straight. Penis large, base rectangular, inner edge without folds; filament short, narrow, tapering, oblique; lobe rather medium-sized, squarish, oblique (Fig. 3A). Terminal gland (Fig. 3A–D) narrow, usually transversely positioned along outer edge of lobe (58/86 specimens examined from three samples), less frequently horizontal (28/86), sometimes borne on short stalk. Penial duct narrow, near outer edge, almost straight. Penial filament having black internal pigment core along most of length. Ovary small (0.75 whorl), composed of simple, stalked lobes; slightly overlapping stomach anteriorly. Female glandular oviduct and associated structures shown in Figure 3E–F. Coiled oviduct narrow, vertical. Bursa copulatrix small, ovate, horizontal, about 50% overlapped by albumen gland. Bursal duct longer than bursa, narrow, opening from distal edge, partly embedded in albumen gland proximally, entirely embedded distally, junction with common duct well in front of posterior wall of pallial cavity. Seminal receptacle small, pouch-like, positioned near ventral edge of albumen gland slightly anterior to bursa copulatrix. Albumen gland largely visceral. Capsule gland composed of two distinct tissue sections. Genital aperture a terminal slit.

Marstonia comalensis is distributed in the upper portions of the Brazos, Colorado, Guadalupe and Nueces River basins, south-central Texas (Fig. 4); almost all of these localities are on the Edwards Plateau. We were unable to confirm a previous report of this species from a drainage canal near Galveston Bay (Cable and Isserhoff 1969). Marstonia comalensis lives in cold water springs near their sources and slack water riverine habitats; it has been most commonly found on mud, aquatic vegetation and dead leaves.

The material referred to Marstonia comalensis herein, which includes specimens from the Guadalupe River above the Comal Creek confluence, closely conforms to the types of this species both in size and shell shape (Fig. 1A–C). This snail clearly belongs to Marstonia based on its strongly wrinkled shell protoconch, distally bifurcate penis ornamented with a gland along the edge of the lobe (terminal gland), and connection between the oviduct and bursal duct well in front of the posterior pallial wall (Thompson and Hershler 2002). As noted above, this generic placement is also supported by molecular phylogenetic evidence (Hershler et al. 2003a).

The original collections of Marstonia comalensis are worn shells having the appearance of drift material (Fig. 1A). We have not seen any live-collected specimens of this species in the numerous samples that we have examined from the type locality (Comal Springs) and other waters near New Braunfels. The various reports of living Marstonia comalensis from this portion of the Guadalupe River basin (e.g., EHA 1975; Arsuffi 1994; Tolley-Jordan and Owen 2008) are probably of misidentified Cincinnatia integra, as evidenced by the illustrations of shells in several of these documents (Lindholm 1979, fig. 4; Cauble 1998, fig. 7). It is possible that Marstonia comalensis became extinct at Comal Springs when this water body temporarily dried in 1964 (USFWS 1996); it is also possible that the shells of this species which have been found at this site were washed downflow from extant populations in the headwaters of the Guadalupe River.

Taylor’s (1975) allocation of Amnicola comalensis to Cincinnatia appears to have been the result of a misidentification as all of the material in his collection that he referred to this species (per the original labels), including several lots from the type locality, is Cincinnatia integra (RH unpublished). Some of these records were detailed in an unpublished manuscript, “Freshwater molluscs from the Nueces River drainage, Texas” that Taylor circulated in the mid-1970’s. Cincinnatia integra, which is widely distributed in Texas (Hershler and Thompson 1996), has been frequently confused with Marstonia comalensis in museum collections despite the obvious differences between their shells that were noted in the original description of the latter (Pilsbry and Ferriss 1906). These two species also well differentiated anatomically (see Hershler and Thompson 1996 for details of the former).