THOMPSON: SNAIL GENUS MARSTONIA


 When F. C. Baker (1926:195) proposed Marstonia as a subgenus of Amni-
cola, he included seven species: Amnicola lustrica Pilsbry, A. gelida F. D.
Baker, A. oneida Pilsbry, A. walker Pilsbry, A. pilsbryi Walker, A. greenensis
F. C. Baker, and A. winkleyi Pilsbry. Of these, A. walker and A. pilsbryi are
species of Lyogyrus (Thompson 1968:162-163); and A. oneida, A. gelida, and
A. winkleyi are here considered objective secondary synonyms of M. lustrica.
A. greenensis is a Pleistocene fossil. Speculation about its generic affinities
is fruitless, because critical anatomical data are not available for such fossil
species. Taylor (1960:50) placed the Pliocene Amnicola crybetes Leonard
1952 in Marstonia, but that allocation is also purely speculative. Thus, M.
lustrica was the only species unequivocably placed in Marstonia until recently
when M. agarhecta was described (Thompson 1969:242-247).

 ACKNOWLEDGMENTS
 Many people have assisted me in this study. To all of them I am grateful for their contribu-
tions in time and thought. For the loan of specimens in their charges I wish to acknowledge:
Kenneth J. Boss, Museum of Comparative Zoology (MCZ); Joseph Rosewater and Joseph P. E.
Morrison, United States National Museum of Natural History (USNM); J. B. Burch and Henry
van der Schalie, Museum of Zoology, University of Michigan (UMMZ); Juan Jose Parodiz,
Carnegie Museum (CM); Alan Solem, Field Museum of Natural History (FMNH); and George
M. Davis, Academy of Natural Sciences, Philadelphia (ANSP). Harold J. Walter, Dayton, Ohio,
provided me with critical anatomical material of M. lustrica. Beverly E. Johnson, Gainesville,
Florida, assisted me in fieldwork in Alabama, Georgia, and Tennessee. Figures 1, 2, and 3A-C
were made by Donna Born Drake and the remaining figures by Nancy Halliday; I am grateful
to both for their skillful contributions. The SEM were made by Sylvia Scudder, Florida State
Museum.
 Field work in the southern states was supported by funds from three agencies: U. S. Depart-
ment of the Interior, Office of Endangered Species (Contract # 14-16-008-785, 1973), Tennessee
Valley Authority (Contract # TV-37778A, 1973), and the University of Florida, Division of Spon-
sored Research (NIH Biomedical Grant 69-25).

 Marstonia F. C. BAKER 1926
Marstonia F. C. Baker 1926:195. (Type species: Amnicola lustrica Pilsbry 1890; see Berry
 1943:29, Thompson 1969:242-243.
 GENERIC CHARACTERISTICS.-This is a genus of small conical or elliptical-
conical snails, in which the shell seldom exceeds 5 mm. The shell, operculum,
and radula are not diagnostic at the generic level.
 The shell is conical to elliptical-conical in shape, with about five whorls at maturity, and is
about 0.46-0.80 times as wide as high. The periostracum is light gray. The peristome is usually
complete across the parietal margin of the aperture, but occasionally it may be incomplete.
Sculpturing either is absent or consists of fine incremental striations. The umbilicus may be
broadly perforate or closed. The embryonic whorls protrude, with the nuclear whorl about
0.25 mm in diameter perpendicular to the initial suture. The embryonic sculpture consists of a
densely folded and anastomosing series of wrinkles and pits, and is similar to that found in other
bilobed American hydrobiid genera (Fig. 4).
 The operculum (Fig. 25) is paucispiral, thin, membranous, transparent, ovate to modified
elliptical in outline, and consists of about 2.5 whorls. The nucleus is near the left and lower third

of the outer face. The radula is taenioglossate with the following formula: C3T--5,