330 STROPHOMENIDA. 
TRopIpOLEpTus, Hall, 1859. 
Etym.— Tropis, a keel, and leptos, thin; the carinated ventral 
valve and shallow visceral cavity, in its analogy with Leptzna. 
Distr.—2 sp. Silurian; United States. Strophomena carinata, 
Conrad (exxxviii, 43, 44). 
Shell transversely oval, or longitudinally semielliptical, articu- 
lating by teeth and sockets, hinge-line about equal to the breadth 
of the shell. Ventral valve convex, with a linear area and trian- 
gular foramen in the margin of the area; from the inner edges 
of this proceed the dental Jamelle, which are separated from the 
area by a narrow groove strongly crenulated on the outer edge, 
and extending obliquely outwards, terminating in a low ridge 
which partially surrounds the muscular impression; dorsal 
valve concave; cardinal process prominent, wedge-shaped, sup- 
porting the bases of the crura; dental fossets crenulated, surface 
plicated ; shell-structure punctated. 
Vivunina, Hall, 1861. 
EHtym.—Vitula, a goddess. 
Distr.—Devonian ; New York. V. pustulosa, Hall (cxxxviil, 
45-48). 
Shell resembles that of Tropidoleptus, but the dental processes 
are not crenulated, nor distinctly separated from the area as in 
that genus. 
STROPHOMENA, Rafinesque, 1827. 
Etym.—Strophos, bent; mene, crescent. 
Syn.—Peridiolithus, Hiipsch, 1768. Brachyprion, Shaler, 
1869. Leptena, Dalman, 1828. 
Distr.—Fossil, 129 sp. Lower Silurian—Carb.; N. America, 
Europe, Thibet. 8S. alternata, Conr. (exxxviii, 49, 50). 
Shell semicircular, widest at the hinge-line, concavo- convex, 
depressed, radiately striated; area double ; ventral valve with 
an angular notch, progressiv ely covered by a convex pseudo- 
deltidium ; umbo- depressed, rarely (?) perforated, in young 
shells, by a minute foramen ; muscular depressions four, central 
pair narrow, formed by the adductor; external pair fan-like, left 
by the cardinal and pedicel-muscles; dorsal valve with a bilobed 
eardinal process, between the dental sockets, and four depres- 
sions for the adductor muscles. 
There are no apparent brachial processes in the dorsal valve of 
Strophomena, and it is possible that the spiral arms may have 
been supported at some point near the centre of the shell as in 
Productus; S. rhomboidalis occasionally exhibits traces of spiral 
arms, in the ventral valve. &. /atissima, Bouch., has plain areas, 
like Calceola. 
The valves of the Strophomenas are nearly flat until they 
