GENUS TROPIDQLEPTUS. 
ms 
Surface plicate : shell-structure punctate. 
The typical species of this genus is a concavo-convex shell, having 
the general form of Lept^ena and Strophomena, and was originally 
described by Mr. Conrad as Strophomena carinata. It differs from all the 
genera of Strophomenidse in both external and internal characters, and, 
for these reasons, has been separated. The shell is externally strongly 
ribbed, and the texture is finely punctate throughout its substance. The 
ventral area is well defined, narrow and linear. The fissure or foramen 
is very large and wide, and is excavated above the area line, coming 
quite up to the beak, and sometimes even including the apex which is 
worn away or absorbed. . 
The teeth, which are a little separated from the margins of the fora¬ 
men and not continuations from it, are strong and thickened below, while 
they are deeply crenulated on the summit and exterior margins. There 
is a narrow low median ridge in the cavity of the valve; and the divari- 
cator muscular impressions are broad, and flabelliform. The occlusor 
muscular impressions have not been satisfactorily observed. 
The dorsal valve has a narrow area, and a wide and strong cardinal 
process which nearly or quite fills the foramen of the opposite valve. 
This process is often simple exteriorly, above the limit of the smooth 
or striated pseudo-deltidium which covers it near the hinge-line; but 
just within the valve it is broadly grooved in the middle, usually with 
two small deep pits just within the external smooth callosity, and on each 
side there is a groove and accessory lobe, frequently not conspicuous. 
The divisions made by the median groove diverge and terminate below 
in obtuse processes which have some similarity with the bases of crural 
processes in Orthis, but have more analogy with the Terebratulidse. These 
processes are sometimes clearly broken at their termination, but are 
often smooth as if the roughened surface had been cicatrized during the 
life of the animal. Below these forks of the process there is a narrow 
median crest or septum which reaches beyond the middle of the valve, 
and sometimes nearly to the front. From the limbs of the thickened 
divergent processes there proceed slender crura which, at first bending 
