GASTEROPODA. 
21 
from the outer margin of the aperture to the opposite side of the volution, is 
more than three inches; the longitudinal diameter of the aperture is nearly 
two inches; the width a little less. 
Formation and localities. In the Upper Helderberg limestone, at Darien, N. Y. 
I have also seen the same from Port Colborne, Canada West. 
PLATYOSTOMA Conrad. 
Platyostoma lineata. 
PLATE X, FIGS. 1-21. 
Platyostoma lineata, Conrad. Jour. Acad. Nat. Sci. Phil., vol. 8, p. 276, pi. 17, f. 7. 1842. 
“ “ “ Hall. Illustrations of Devonian Fossils : Gasteropoda, pi. 9. 1876. 
Shell subovate, approaching to subglobose. Spire elevated above the body- 
whorl, though varying in degree; in some extreme varieties, on the same 
plane or below the outer volutions. The shell with four or five volutions 
when entire, but seldom preserving more than three,—the apex being 
usually imperfect. The outer volution usually very ventricose and regu¬ 
larly convex, a little depressed below the suture-line (but not canaliculate). 
Aperture suborbicular in perfect specimens, sometimes subrhomboidal; 
outer lip thin, with a sharp entire margin; columellar lip thickened, 
folded, and reflexed over the umbilicus, which in adult specimens, is 
entirely closed. 
Surface marked by fine, nearly equidistant, thread-like revolving striae, which 
are cancellated by fine concentric striae of about the same strength, but 
unequally distant; the latter sometimes bent abruptly backwards upon 
the back of the shell, indicating a sinus in the lip at some period of 
growth, and are frequently crowded in fascicles giving a rugose character 
to the surface. 
In well-preserved specimens, the surface is beautifully cancellated; and in 
