THOMPSON: SNAIL GENUS MARSTONIA


4a. Shell thinner, transparent or translucent; openly umbilicate ... .......... ... .......... 6
5. Shell ovate-conical in shape, spire convex in outline; outer lip straight in lateral
 profile; suture shallow, whorls not shouldered. Verge with two small glands on
 apical lobe and a small raised gland near base of verge. Creeks, Limestone Co.,
 A lab a m a ... ........... ..... ..... ... ... ..... ........................................................... p a ch y ta n sp .
5a. Shell nearly conical, spire straight sided; outer lip strongly curved in lateral pro-
 file; suture deep, whorls shouldered. Verge with a single large gland on the apical
 lobe. Tennessee River, Alabama .. ....... arga n. sp.
6. Shell conico-terete in shape; suture deep, producing strongly shouldered whorls;
 outer lip slightly arched forward in lateral profile. Owens Spring, Sequatchie,
 Marion Co., Tennessee ...................... .. . ogmorhapha n. sp.
6a. Shell ovate-conical or elongate-terete in shape; suture shallow, whorls not
 shouldered; outer lip straight in lateral profile ... .. lustrica (Pilsbry), olicacea (Pilsbry)


 Marstonia arga new species
 DIAGNOSIS.-The verge tapers basally and bears a single apical gland on the apical lobe. The
glandular development of its verge is similar to M. lustrica and M. ogmnorhapha. It further re-
sembles the latter in having a reniform prostate. It differs anatomically from other Marstonia
species in having the bursa copulatrix completely overlapping the posterior pallial oviduct, and
in having the distal half of the bursal duct exposed.
 The shell is distinct in being imperforate or at the most narrowly rimate, in having a coni-
cal spire, in having a thick callused ridge inside the aperture, and in having a strongly curved
lip in lateral profile.
 SHELL (FIGS. 1A-B; 6A-F).-The shell is medium-sized and conical, with a relatively large
body whorl, and is about 0.52-0.63 times as wide as high. The spire is acutely conical, nearly
straight-sided, and over half the length of the shell. The aperture is about 0.36-0.50 times the
length of the shell. The shell is very thick and opaque or weakly translucent, with a heavy
callused ridge inside the aperture. The umbilicus is weakly rimate or imperforate. The color is
light grayish-brown. The surface is smooth and dull in most specimens. Occasional specimens
have strong, widely spaced vertical -striations and light zones that correspond to growth rest
stages. There are 4.6-5.4 whorls in large specimens (4.7 in holotype). The suture is moderately
impressed. The whorls tend to be flattened along the periphery. The apical whorl is about
0.25 mm in diameter, perpendicular to the initial suture, and projects above the succeeding
whorl. The aperture is irregularly ovate in shape. It is about 0.86-0.96 times as wide as high, and
may be slightly solute in gerontic specimens. The posterior corner is weakly angulate or bluntly
rounded. The peristome extends completely across the parietal wall at maturity. The outer lip
is strongly arched forward in lateral profile, projecting considerably anterior to the posterior
corner and the basal margin. The basal lip is recurved slightly posterior to the columellar margin.
 Measurements in mm of 10 paratypes selected to show maximum variation are as follows
(measurements of the holotype are in parentheses): shell length 3.2-3.9 (3.3), shell width 2.1-
2.5 (2.3), aperture length 1.4-1.6 (1.5), aperture width 1.2-1.5 (1.4).
 FEMALE (FIG. 7B-C).-The lower female reproductive system is similar to that of M. lus-
trica, except for the following features. The glandular wall of the oviduct loop is comparatively
thin. The seminal receptacle opens directly into the oviduct I and lacks a distinct duct. The
bursa copulatrix projects completely beyond the posterior pallial oviduct. The bursal duct is
exposed for about half its length below the bursa on the mesad surface of the posterior pallial
oviduct.
 MALE (FIG. 7A).-The vas deferens I is strongly convoluted above the prostate. The latter
is about twice as long as wide and is roughly kidney-shaped. The vas deferens II is only slightly
convoluted within the coelomic wall and is nearly straight within the verge and penis.
 The verge is relatively long and is tapered basally. It bears a single large gland confined to
the tip of the apical lobe. The penis is relatively long and slender.
 OPERCULUM (FIG. 25, B).-The operculum is broad and irregularly ovate, thin, transparent,
and slightly amber colored. It is paucispiral, with about 2.5 whorls. The nucleus is in the lower
and right third of the face. The outer surface is weakly striate, with incremental striations.


1977