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. 
Platyceras 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, 
