198 AMERICAN JOURNAL 



terminal ; area slightly depressed below the submargins, exca- 

 vated beneath the beaks and umbones ; posterior side subcunei- 

 form ; disk convex in the middle, posteriorly sloping in a nearly 

 straight line to the end margin ; extremity rounded, situated nearer 

 to the summit than to the ventral margin. 



This is the largest and most ventricose species, remarkable for 

 having the shell silicified. The internal mould is of indurated 

 ferruginous clay. 



Plate 11, fig. 7. Outline of Dysis gracilis,^ enlarged. 

 Plate 11, fig. 8. Outline of Iscea Ortoni, enlarged. 



