GENUS TROPIDOLEPTUS. 



281 



termination of the crural process, and on each side of the median ridge, 

 are striated ; and this striatiou often extends in a wide flabelliform expan- 

 sion, probably due to vascular impressions. Towards the margin, the interior 

 of both valves is strongly pustulose. 



The accompanying wood-cuts illustrate the parts referred to above. 



Fig. 5. 

 Interior of the dorsal valve. 



Fig. 6. 

 Longitudinal section of the dorsal valve . 



j. Cardinal process. 



b. Crenulated teeth-sockets. 



c. Crural processes. 

 /. Loop. 



s. Septum. 



j. Cardinal process. 



b. Tooth-socket. 



c. Crural processes. 

 I. Loop. 



s. Septum. 



In the punctate texture of this shell, it differs from either of the G-enera 

 Lept^na, Strophomena or Strophodonta; but this might not be an 

 objection to admitting Tropidoleptus into the family, were the other 

 characters coincident. The area is longitudinally striated, and presents a 

 different aspect from any of the Strophomenid^, but has analogy with 

 some of the Orthides. The teeth are not extensions of the lamellae bound- 

 ing the foramen, but distinct from it and deeply crenulate or lobed, and 

 inserted into corresponding crenulate sockets in the dorsal valve. The 

 form of muscular impressions, so far as known, is not very dissimilar to 

 those of Strophomena or Orthis. 



In comparing the form of the cardinal process and its appendages, we 

 shall find it almost entirely similar to that of Leptoc(ELIa, as shown in 

 two authentic species {L. Jlahellites and L.Jimbriata), and the mi||gcular 

 impression of the ventral valve is quite like that of the same species. 



The exterior extremity of the cardinal process presents considerable 

 variety of aspect, when a large number of individuals are examined. In 

 some of them this part, if stripped of the external callosity or pseudo- 

 deltidium, would have the main process bilobed, with a sinus a little below 

 the apex, and an accessory lobe on each side, similar to some of the species 

 of Productus. 



[Assem. No. 239.] 



36 



