PALAEONTOLOGY OF NEW-YORK. 



In the preceding arrangement of the materials in the collections 

 forming the subjects of this volume, there has been comparatively little 

 difficulty in assigning to each genus its approximate relations; and from 

 the Orthides onward, there is a very natural succession among the genera. 

 In this arrangement, however, two genera, Tropidoleptus and Vitulina, 

 are left out of the series ; nor does there appear to be any place where 

 either one of these can be naturally introduced. 



We had originally supposed that Tropidoleptus would find its place 

 among the Strophomenidae ; but there have been at all times some 

 important objections to placing it in this relation, while later discoveries 

 have rendered such a reference unnatural. Notwithstanding the con- 

 cavo-convex form, area, and large fissure under the beak of the ventral 

 valve, still the punctate structure of the shell and the character of the 

 crura seem to indicate its relations to be with the Terebratulidas ; and 

 I have accordingly placed it in an order following the authentic genera 

 of this family. 



Gekls Tropidoleptus (Hall). 



[ Gt . Tpom(, carina ; J-ctttoj-, <c«u<«.] 



TBOPiDCUprca : Hail, Report on the State Cabinet. 1857. 

 •• Id. Paltcozoic Fossils. 1857. 



" Ib. Twelfth Report on the State Cabinet; p. 31. 1859. 



Shell transversely suboval or semielliptical, concavo-convex : hinge- 

 line extended, not crenulate ; articulating by teeth and sockets. 



Ventral valve convex, vfith a distinct area, and wide fissure beneath 

 the beak. Dental lamella9 distinct from the margin of the fissure, 

 crenulate. 



Dorsal valve concave, with crenulate dental fossets; a strong cardinal 

 process, with diverging lobes in the interior, which support slender 

 crura that converge to and unite with the median crest. 



