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, Coxrad. 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 



