GASTEROPODA. 19 



Surface marked by concentric undulating lamellose striae ; shell very thick. 



The length of large specimens is two and a half inches, and height a little 

 less than two inches. The transverse diameter of the aperture is from 

 one and a half to two inches, and the longitudinal diameter a little less. 



This shell is remarkable for its great strength and thickness ; it is more 

 oblique than the P. rarispinum and expands less rapidly, and the fold or 

 carina near the summit is on the left side. 



Formation and localities. In the Upper Helderberg limestone, Helderberg 

 mountains in Albany and Schoharie counties, N. Y. 



Platyceras Argo. 

 plate viii, figs. 1-6. 



Platycerax Argo, Hall. Descriptions of New Species of Fossils, etc., p. 11. 1861. 

 " " Fifteenth Rep. N.Y. State Cab. Nat. Hist., p. 39. 1862. 



" " Illustrations of Devonian Fossils : Gasteropoda, pi. 8. 1876. 



Shell varying from subdiscoid to obliquely subovoid, with body- whorl extremely 

 ventricose. Nucleus minute, with the apex closely enrolled for about two 

 volutions, beyond which it expands more or less abruptly ; the last volu- 

 tion nearly or quite in contact with the preceding one. The body of the 

 shell is rounded above, and often obtusely subtrilobate below the middle, 

 becoming rounded towards the aperture. Aperture round or round-ovate, 

 sometimes approaching to quadrangular, broadly sinuate on the right 

 side, and deeply sinuate on the left side, where the peristome is sometimes 

 strongly reflexed, forming an apparent columellar lip. The shell is thick, 

 and its structure is lamellose, as in the Cephalopoda, with a nacreous 

 lustre. 



Surface marked by fine revolving striae, with distant stronger ones, and can- 

 cellated by scarcely coarser concentric undulating striae, which are bent 

 backwards upon the somewhat regular ridges, presenting several bands 

 similar to the single one in Pleurotomaria. 



This species is remarkable for the peculiar lamellose structure throughout, 



