404 
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 Terebratulidge; and 
I have accordingly placed it in an order following the authentic genera 
of this family. 
Genus Tropidoleptus (Hall). 
[Gr. rpomc, carina; Xenroc, tenuis.'] 
TaopiDCLBPrus : Hall, Report on the State Cabinet. 1857. 
« Id. Palasozoic Fossils. 1857. 
<< Id. 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, with a distinct area, and wide fissure beneath 
the beak. Dental lamellas 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. 
