336 MEMOIRS OF THE CARNEGIE MUSEUM 
c. Shell ovate or elongate; beaks not very prominent, and not much produced; color of epidermis 
brown to black, only indistinctly rayed, if at all; nacre varying from white to deep purple, with 
the dark shades prevailing. Genus Elliptio Rafinesque .? 
2. Subfamily AnopontTIN@ Ortmann. 
Inner lamina of inner gill free from the abdominal sac or more or less connected 
with it, rarely entirely connected; supra-anal opening well separated from the anal, 
sometimes the connection of the mantle separating it from the anal is very long, 
and the supra-anal is quite short; branchial opening well defined, no papillae or 
flaps in front of it on the edge of the mantle; marsupium formed by the outer gills 
in their whole length, distending, when charged, and the thickened tissue at the 
edge capable of stretching out in a direction transverse to the gill, but not beyond 
the edge (or only slightly so); water-tubes in the gravid female divided longi- 
tudinally into three tubes, with only the one in the middle used as an ovisac, and 
closed at the base of the gill; glochidium semicircular or triangular, with a spine 
(hook) in the middle of the ventral margin of each valve; shell generally not 
very heavy, often thin, never round, but more or less elongated; color of epidermis 
generally bright and with color markings; sculpture of the beak double-looped or 
concentric, in the latter case often extremely heavy; hinge rarely complete, and 
if so, of peculiar structure; generally there is a distinct tendency toward the re- 
duction of the hinge-teeth, and often they are completely absent; sexual differences 
in the shell very rarely present. 
GENERA 
a. Sculpture of the beak concentric, heavy; inner lamina of inner gills having a tendency to become united 
with the abdominal sac. 
bt. Shell with cardinal teeth; lateral teeth present or wanting; ovisac of each water-tube in the gravid 
female not subdivided; placentse poorly developed, and glochidia practically free. 
Genus Alasmidonta Say. 
6. Shell with the hinge-teeth obsolete; only a mere vestige of the cardinal teeth remaining; ovisac of each 
water-tube subdivided into a number of compartments, each containing a well developed placentula, 
running crosswise to the gill; placentule persistent till they are discharged. 
Genus Strophitus Rafinesque. 
a, Sculpture of the beak generally double-looped, if concentric, not heavy; inner lamina of inner gills always 
free; ovisacs not subdivided. 
b', Shell rather solid, lateral hinge-teeth more or less imperfect; cardinal teeth present, but of peculiar 
shape, sculpture of the beak more or less double-looped. Genus Symphynota Lea, 
&?, Shell thin; hinge edentulous, or with mere traces of cardinal teeth. 
23See above p. 330, footnote. The European genus Unio Retzius, although similar in anatomical detail, differs in shape 
and color of shell, and chiefly in beak sculpture (wavy, zig-zag or double looped) and glochidia. The type species of Ellip- 
tio is U. crassidens Lamarck. 
