THOMPSON: SNAIL GENUS MARSTONIA


The shell is relatively thin and translucent. The umbilicus is open, moderately wide, and is never
obscured by the peristome. The aperture is 0.41-0.46 times the height of the shell. The spire is
slightly convex in outline. There are 5.0-5.4 whorls (5.4 in holotype), which regularly descend
to the aperture. The apical whorl projects above the succeeding whorl and is 0.25 mm in diam-
eter. The whorls are strongly arched with a moderately impressed suture but are sloped and not
shouldered. The penultimate whorl is slightly rotund. The postembryonic sculpture consists of
fine, close, irregular incremental striations. Occasionally the striations are intensified about the
umbilicus or may be folded into weak thread-riblets, as in the lectotype (Fig. 1, D). The aper-
ture is broadly ovate in outline, is weakly angulate at the posterior corner, and is about 0.83-
0.90 times as wide as high. The plane of the aperture lies at an angle of about 150 to the axis of the
shell. The peristome is usually incomplete across the parietal wall, except in very large individ-
uals. The peristome is very thin, except at the baso-columellar angle, is weakly reflected along
the columella, and is straight in lateral profile.
 Measurements in mm of specimens with five or more whorls holotypee measurements in
parentheses) are as follows: shell length 3.9-4.5 (4.35), shell width 2.6-3.0 (2.75), aperture height
1.7-2.0 (1.8), aperture width 1.5-1.7 (1.5).
 OPERCULUM (FIG. 25F).-Thin, membranous, light yellow in color, and transparent; pauci-
spiral, broadly ovate in outline, and consists of about 2.5 whorls. The nucleus is located in the
lower quarter and slightly to the left of the midline. The outer surface bears a few fine incre-
mental striations.
 VERGE.-Aspects of the soft anatomy are unknown in this species.
 TYPE LOCALITY.-[Big Spring Creek], Huntsville, Madison Co., Alabama. LECTOTYPE
by present designation: ANSP 65466a, collected by H. E. Sargent, 1894. PARATYPES: ANSP
65466 (456 specimens); same data as lectotype. The lectotype is a specimen selected by Pilsbry
as the type but apparently never designated as such in print.
 RANGE (FIG. 15).-Apparently this species was confined to Big Spring
Creek, which drains south into the Tennessee River. Goodrich (1944:7) stated
that the species was common in streams and springs in and about Huntsville,
but no specimens are available in museum collections other than from Big
Spring Creek. Goodrich (1939:130, 1944:7) also recorded this snail from the
Ogeechee River system in Georgia and the Coosa River system in Alabama,
but his records from both drainages are misidentifications of other species.
 This snail probably is extinct. The creek is badly polluted and has been
channelized for most of its course. No specimens were found by the author
during two visits to Big Spring Creek during 1973.
 REMARKS.-The specific status of this entity is questionable. Its shell differs
from M. lustrica in having a more inflated penultimate whorl in large speci-
mens, a broader operculum, and a darker periostracum. If M. olivacea was
from a more northern locality, I would be tempted to consider it a synonym
of the highly variable M. lustrica. However, extenuating circumstances dis-
suade me from doing so.
 The type locality of M. olivacea is disjunct to the south from the known
geographic range of M. lustrica by nearly 400 miles. This geographic isolation
may not be so great as available data suggest, since almost nothing is known
about the hydrobiid fauna of Kentucky and western Tennessee. On the other
hand, I have collected extensively in the Duck and Buffalo rivers, and in por-
tions of the Cumberland River system in Tennessee without finding Mar-
stonia. It remains to be demonstrated whether or not the genus occurs in lower
courses of the Tennessee River and in western Kentucky and Tennessee.