BULLETIN FLORIDA STATE MUSEUM


 Marstonia castor new species
 DIAGNOSIS.-Marstonia castor is a minute snail, less than 2.5 mm long, ovate-conical in shape,
 relatively thick and solid, and has about 3.9-4.4 rounded whorls with a deeply impressed suture.
 The peristome is complete at maturity, and the umbilicus is narrowly perforate. The verge has a
 tubercular gland on the mesad surface in addition to the apical gland.
 SHELL (FIGs. 3C, 21A-D).-The shell is minute, ovate-conical in shape, and about 0.66-0.73
 times as wide as high. It is narrowly but distinctly umbilicate. The circum-umbilical area is almost
 obtusely angulate. The shell is moderately thick for its size, semi-transparent, and light gray in
 color. The spire is slightly convex in outline and slightly longer than the aperture, which is about
 0.44-0.49 times the length of the shell. There are 3.9-4.4 Whorls in mature shells. Females tend
 to be slightly larger than males and have about 0.3 more whorls. The apical whorl is raised and
 rounded with a well-impressed suture, and is about 0.23-0.24 mm in diameter perpendicular
 to the initial suture. Post-apical whorls have a deeply impressed suture, are strongly and evenly
 arched, and have numerous fine incremental striations. The aperture is ovate in shape and about
 0.81-0.91 times as wide as high. In lateral profile the plane of the aperture lies at about 18-20
 to the axis of the shell. The peristome is complete across the parietal margin at maturity and is
 neither advanced nor receded in lateral profile. The outer lip is sharp, nearly rounded to the
 columella, and may develop a relatively thick internal callus. The parietal margin of the aper-
 ture is nearly straight.
 Measurements in mm of six adult specimens selected to show the range of variation are as
 follows holotypee measurements in parentheses): shell length 1.94-2.70 (2.30), shell width 1.40-
 1.98 (1.51), aperture height 0.94-1.33 (1.05), aperture width 0.83-1.08 (0.86).
 OPERCULUM (FIG. 25E).-Broadly ovate in shape, paucispiral, and consisting of about two
 whorls, with the nucleus located in the lower left quarter. Thin, membranous, transparent. The
 outer surface shows a few fine incremental striations.
 VERGE (FIG. 22A-B).-The verge is relatively slender and has a relatively enlarged penis,
 which is about twice the length of the apical crest. The apical crest is nearly completely covered
 by an apical gland. On the ventral side near the middle of the verge is a small globular gland
 raised on a low pedicel.
 TYPE LOCALITY.-Cedar Creek, 3.4 mi. S-SW Coney, Crisp Co., Georgia. HOLOTYPE: UF
 22176; collected 19 November 1973 by Fred G. Thompson. PARATYPES: UF 22177 (51); same
 data as the holotype.
 In addition to the type series, I have examined about 300 other specimens
from the type locality (UF 22178).
 The type locality is a small clear stream that flows through a cypress
slough and drains into the east side of Lake Blackshear, a reservoir on the Flint
River. Snails were generally distributed in the creek on Najas and to a lesser
extent on clumps of Chara in shallow clear water that had only a slight cur-
rent. They were not found on the substrate or debris. At no place was the snail
abundant.
 RANGE (FIG. 20).-This snail is known only from the type locality.
 ETYMOLOGY.-The species epithet castor is taken from the generic name of
the beaver, Castor canadense, with which it shares the type locality.

 Marstonia agarhecta THOMPSON
MAarstonia agarhecta Thompson 1969:243-247; fig. IA-F.
 DIAGNOSIS.-This is a minute snail most closely related to Al. castor by the structure of the
verge, which bears a single enlarged epidermal gland on the apical lobe but lacks other glands.
The penis is relatively large and about twice the length of the apical lobe. The shell is minute
(about 2.3-2.7 mm long), conical, and very fragile. It has an incomplete peristome and about 4.4-
4.6 whorls that are separated by a deeply impressed suture.
 SHELL (FIGs. 3B, 23A-D).-The shell is minute, very thin and fragile, transparent, conical,


Vol. 21, No. 3