BracMopoda of the Cincinnati Group. 51 



not it was identical with Strophomena sinuata (Emmons). After care- 

 fuUv examining the subject, I have no doubt that Dr. Emmons 

 obtained his specimens from Cincinnati, as he says he did, and that he 

 figured and described this species. 



Genus Strophomena — (Eafinisque, 1827). 



[Etym.—Sfropkos, bent; mene, crescent.] 



Sliell semi- circular, usually widest at the hinge line, concavo-convex, 

 depressed, radiately striated ; area double ; ventral valve with an 

 angular notch, progressively covered by a convex pseudo-deltidium ; 

 umbo depressed, perforated by a minute foramen ; muscular depressions 

 four, central pair narrow, formed by the adductor; external pair fan- 

 like, left by the cardinal and pedicle muscles; dorsal valve with a 

 bilobed cardinal process between the dental sockets and four depres- 

 sions for the adductor muscle. 



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 center of the shell, as in Prodticta. 



Strophomena alternata — (Conrad, 1838). 



Shell varying in size from three fourths of an inch in length to two 

 inches or more, semi-oval, breadth usually greater than the length ; 

 hinge line usually longer than the breath of the valve further forward ; 

 lateral extremities usually compressed and deflected ; lateral margins 

 roundins: forward to the front in a semi circular outline, or sometimes 

 rounding in the middle, so as to present a subtrigonal outline. 



Dorsal valve flattened in the urabonal and cardinal regions; more or 

 less concave in the central and anterior portions, and more or less 

 curved upward around the anterior and lateral margins ; beak small, 

 projecting slightly beyond the sublinear area, which is directed nearly 

 backward. Interior with bifid, strong, cardinal process, diverging 

 obliquely forward, flattened and longitudinally striated on the pos- 

 terior faces ; sockets for the reception of the teeth of the other valve 

 well defined ; socket ridges very small and uniting behind the cardinal 

 process, to form a kind of ftilse deltidium ; muscular scars compara- 

 tively small and deeply impressed immediately in front of the cardinal 

 process, on each side of a small, short, mesial ridge, and nearly 

 surrounded by a low, obtuse ridge, formed by a thickening of the valve, 

 and more or less longitudinally and irregularly furrowed ; anterior and 



