330 PALiEONTOLOGY OF NEW-YORK. 



The name Orthonychia ( proposed in the Report of the Fourth Geo- 

 logical District in 1843), for certain forms of this group of fossils, would 

 apply to some of the species among those of the Lower Helderberg rocks, 

 in which the body of • the shell is straight or curving, gradually diminish- 

 ing above, arched or in some degree spiral at the apex, with the last 

 volution or more quite free. 



The gradations, however, are so imperceptible, that it is scarcely 

 possible to separate any collection of species that do not embrace other 

 than these forms alone ; since on the one side they pass to those having 

 the spire closely enrolled with one or more volutions, and, on the other, 

 to those with a simply arcuate apex, and thence to the species which 

 are essentially straight. For the straight forms with cancellated surfaces, 

 and often with the addition of longitudinal plications, I had proposed 

 the name Iqoceras ; and should there prove to be a sufficient reason for 

 the separation, this name may be adopted. 



The species with a single small volution at the apex, and the volutions belQW 

 elongate, straight or subspiral, are arranged principally on Plate lxiij. 



Platjceras laiiiellosuiii ( n. s.). 



Plate LXIII. Fig. 1, 2, 3. 



Shell consisting of a single small volution at the apex, which is nearly 

 in the same plane, and from which the shell gradually enlarges and 

 extends, at first in a gradually oblique direction, and then bends sud- 

 denly downwards, making about another volution : aperture expanded, 

 transversely broad-oval. 



Surface with strong transverse imbricating lamellose ridges, the spaces 

 between which are marked by fine longitudinal striae. 



This species differs conspicuously from any other at present known to me in these 

 strata, in its sharp lamelliform ridges, as well as in the general form of the shell. 



