THOMPSON: SNAIL GENUS MARSTONIA and about 0.64-0.74 times as wide as high. It is dimorphic in shape. Females tend to be broader than males. The spire is about equal to the aperture in height and nearly straightsided. The um- bilicus is open but narrowly perforate. There are 4.4-4.6 whorls in adult shells. The apical whorl is 0.23-0.25 mm in diameter perpendicular to the initial suture; protrudes above the succeeding whorl; is strongly rounded with a deep suture; and has very fine, close microscopic striations. The following whorls are strongly rounded with a deeply impressed suture. The body whorl is weakly shouldered. The sculpture on the lower whorls consists of very fine, close incremental striations that are equal in intensity over the surface of the whorls. The aperture is ovate, about 0.77-0.83 times as wide as high, and about 0.45-0.52 times the height of the shell. The greatest diameter of the aperture lies at about 300 to the shell axis. The plane of the aperture is slightly oblique to the shell axis in lateral profile. The peristome is incomplete across the parietal wall. The parietal callus consists only of a very thin glaze. The columellar lip is concave, very thin, and weakly reflected. The outer lip is weakly arched forward above the periphery and slightly recessed near the base. Measurements are given in mm for large specimens as follows holotypee measurements in parentheses): shell length 2.3-2.7 (2.65), shell width 1.6-1.95 (1.75), aperture height 1.1-1.3 (1.2), aperture width 0.9-1.05 (0.95). OPERCULUM (FIG. 25H).-Broadly ovate in shape, with a slight indentation along the parietal margin. It is very thin, membranous, and colorless or only slightly tinged with yellow. It is pauci- spiral with about 2.5 whorls. The nucleus is large and located in the lower left third of the face. The outer surface is sculptured with a few fine incremental striations. VERGE (FIG. 24A-B).-The verge is stout and slightly compressed dorso-ventrally. The distal left margin terminates in an apical lobe that bears an enlarged apical gland. The penis is rela- tively slender, long, and enlarged. RADULA.-Minute, about 450 u long and contains about 42-46 transverse rows of teeth (10 specimens examined). The central tooth is about 19 /L wide and bears an enlarged mesocone with 3-5 ectocones and a single basocone on each side. The lateral tooth bears 2-1-4 to 3-1-5 cusps. TYPE LOCALITY.-Bluff Creek, 10.4 mi. S-SE Hawkinsville, Pulaski Co., Georgia. HOLO- TYPE: UF 20528; collected 31 January 1969 by Fred G. Thompson. PARATYPES: UF 20529 (296); same data as the holotype. Specimens were found in clear water with only a slight current, predomi- nantly in diatomaceous ooze on top of old submerged logs. Occasionally indi- viduals were also found in silt that contained large amounts of diatoms. RANGE (FIG. 20).-This species is known only from the type locality. REMARKs.-Though very different in shell structure, this snail is most closely related to M. castor. Both species are alike in having an enlarged, elon- gate penis and a relatively enlarged and protruding apical lobe. In these re- spects the two species differ remarkably from other Marstonia. LITERATURE CITED Baker, F. C. 1926. Nomenclatorial Notes on American Fresh Water Mollusks. Trans. Wisconsin Acad. Sci., Arts, Let. 22:193-205. 1928. The Freshwater Mollusca of Wisconsin, Pt. 1. Gastropoda. Bull. Wisconsin Geol. Nat. Hist. Surv. 70:i-xx, 1-507, pls. 1-28. Berry, E. G. 1943. The Amnicolidae of Michigan: Distribution, Ecology, and Taxonomy. Misc. Publ. Mus. Zool., Univ. Michigan 57:1-68. Boeters, H. D. 1970. Die Gattung Microna Clessin, 1890 (Prosobranchia, Hydrobiidae). Archiv fur Molluskenkunde 100:113-145. 1971. Pseudamnicola Paulucci, 1878 und Mercuria n. gen. (Prosobranchia, Hydro- biidae). Archiv fur Molluskenkunde 101:175-181. 1974. Westeuropaische Hydrobiidae, 5. Horatia Bourguignet, Plagigeyeria Tomlin und Litthabitella Boeters (Prosobranchia). Archiv fur Molluskenkunde 104:85-92.