BULLETIN FLORIDA STATE MUSEUM


L(2-4)-1-(3-5) M's (15-20). The basocones rise from the lateral angle of the tooth. The stomach
lacks a tube-shaped caecal appendage on the esophageal end.
 The foot is broadly rounded posteriorly and truncate anteriorly. It moves over the substrate
by a gliding ciliary movement. A deep mucous slit overlies the anterior margin. A suprapedal
fold and an external omniphoric fold are absent. A short, weak fold underlies the anterior half
of the operculigerous lobe on each side. The eyes lie at the base of the tentacles and are raised.
The snout, foot, and nape may be pigmented to varying degrees with melanin. Usually the
mantle bears two diffuse black stripes over the mantle cavity. One stripe overlies or parallels
the left margin of the intestine, and the other overlies or parallels the left margin of the gill. The
two bands usually unite over the pericardium. Occasionally the bands are broader and almost
completely cover the mantle cavity and the posterior viscera. The central nervous system is un-
pigmented.
 The male reproductive system of M. lustrica is illustrated in Figure 5A. The verge originates
on the nape beneath the mantle, just to the right of the middorsal line, and is innervated by the
right pleural ganglion. The verge is bilobed distally, and bears a large apical lobe, a short
terminal penis, and a single duct, the vas deferens. The apical lobe bears a single enlarged apical
gland, although some species have an additional smaller gland near the base of the verge. The
glands are apocrine. The gonad consists of 3-5 large lobes. The vas deferens I is strongly con-
voluted near its middle, at which point the vas wall is thickened and glandular. The prostate is
fan-shaped or reniform and overlaps the posterior end of the mantle cavity. The vas deferens HI
is simple and is a closed duct from the prostate to the tip of the penis. A diverticulum to the
mantle cavity is absent from either the vas deferens I or II.
 The female reproductive system of M. lustrica is illustrated in Figures 6B, C. The ovary con-
sists of three primary lobes, each of which may be divided into 2-4 secondary lobes. Each lobe
is the diameter of one primary oocyte. The oviduct I is relatively short and thick, becoming nar-
rower anteriorly. Just posterior to the wall of the mantle cavity the oviduct forms a large in-
verted U-shaped loop along, and transverse to, the middle of the posterior section of the pallial
oviduct. The wall of the loop along this U-segment is very thick and glandular. (This glandular
loop may serve as an albumen gland.) The single seminal receptacle is small, saculate, lies along
the ventral margin of the posterior pallial oviduct, and enters the end of the oviduct loop by a
very short duct. The seminal receptacle is homologous to the seminal receptacle 2 of Radoman
(1973). The bursa copulatrix overlies the posterior mesad surface of the posterior pallial oviduct.
It has a relatively long duct that is usually imbedded in the latter structure. The bursal duct
enters the oviduct I just posterior to the anterior pallial oviduct. A short gonopericardial duct
(GPD) connects the oviduct I to the pericardium. Neither a receptacular duct nor a bursal duct
(which would connect either of these structures to the posterior mantle cavity) is present. The
oviduct II is closed within the anterior pallial oviduct. The female opening is a short vaginal
slit. A short omniphoric groove connects the vaginal slit to the anterior margin of the mantle
collar.
 GEOGRAPHIC DisTRIBUTrrION.-Marstonia is widely distributed in eastern North America, al-
though it does not have a continuous range in that area. One species, M. lustrica, occurs through-
out the St. Lawrence River system, the Great Lakes region, and the upper Mississippi River
system. A disjunct group of species is found in the Tennessee River system in northern Alabama
and adjacent Tennessee and in the Coosa River of Alabama. Three isolated species occur in the
Flint, Ocmulgee, and Ogeechee river systems in Georgia. Marstonia probably occurs in other
rivers in Georgia and Alabama.
 The present distribution of Marstonia strongly suggests that the genus originated in the Ten-
nessee River system and dispersed from there. This interpretation is based on the species diver-
sity presently existing in the Tennessee River. The species there comprise a natural group, from
which the more northern species, M. lustrica, presumably was derived during the Wisconsin
Glacial Period. A second lineage invaded the coastal drainages of Georgia, probably via the
Savannah River System, and evolved as a strongly differentiated species group. The time at which
the coastal invasion occurred is not clear, but it probably was during the Pliocene or very early
Pleistocene.
 RELATIONSHIPS.-Marstonia, as is true of other hydrobine snails, has a
single duct (the vas deferens) within the male reproductive appendage. The
genus belongs to a group that also includes the North American Nymphophila


Vol. 21, No. 3