CONTRIBUTIONS TO PALEONTOLOGY. 



The occlusor muscular impressions have rarely been seen with any degree 

 of distinctness ; but the depressions just at the termination of the crural 

 processes, and on each side of the median ridge, are striated; and this 

 striation often extends in a wide flabelliform expansion, 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 

 same ; j, cardinal process ; J, teeth-sockets ; c, crural processes ; /, loop ; 

 s, septum. 



In the punctate texture of this shell it differs from either of the Genera 

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

 objection to admitting Tropidoleptus into 

 the family were the other characters coin- 

 cident. The area is longitudinally striated , 

 and presents a different aspect from any of 

 the Strophomenidse, but has analogy with 

 some of the Orthides. The teeth are not 

 extensions of the lamellse bounding the 

 foramen, but distinct from it and deeply 

 crenulate or lobed, and inserted into corre- 

 sponding 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 Leptocoelia, as shown in 



FiQ g two authentic species (Z. flabel- 



s / f lites and L. fimbriata), and the 



muscular impression of the ven- 

 tral 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. 



