GENUS TKOPIDOLEPTUS. 405 



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 Strophomenidaa in bothi 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 TerebratulidaB. 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 



