330 STROPHOMENID^. 



Tropidoleptus, Hall, 1859. 



Etym. — Tropis^ a keel, and leptos, thin ; the carinated ventral 

 valve and shallow visceral cavity, in its analogy with Leptsena. 



Distr. — 2 sp. Silurian ; United States. Strophomenacarinata, 

 Conrad (cxxxviii, 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 lamellae, 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. 



YiTULiNA, Hall, 1861. 



Etym. — Vitula^ a goddess. 



Distr. — Devonian ; New York. V. pustulosa, Hall (cxxxviii, 

 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, 182*7. 



Etym. — Strophos^ bent ; mene, crescent. 



Syn. — Peridiolithus, Hiipsch, 1768. Brachyprion, Shaler, 

 1869. Leptsena, Dalman, 1828. 



Distr. — Fossil, 129 sp. Lower Silurian — Carb. ; N.America, 

 Europe, Thibet. S. alternata^ Conr. (cxxxviii, 49, 50). 



Shell semicircular, widest at the hinge-line, concavo-convex, 

 depressed, radiately striated ; area double ; ventral valve with 

 an angular notch, progressivelj^ 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 

 cardinal 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. rhomhoidalis occasionally exhibits traces of spiral 

 arms, in the ventral valve. S. latissivia, Bouch., has plain areas, 

 like Caleeola. 



The valves of the Strophomenas are nearly flat until they 



