PENTAMERID^ OF LOWER DEVONIAN AGE. 381 



Gypidula Iseviascula (n. s.). 



PLATE LVIII. 



Shell varying from subcircular to broadly ovoid, gibbous in the young 



state, becoming ventricose. 

 Ventkal valve, in the young shell, regularly convex, with beak abruptly 

 pointed and neatly incurved, becoming more gibbous or ventricose as 

 the shell increases in size, ■ipith the umbo prominent and beak arcuate ; 

 regularly curving to the sides and front, without mesial sinus or 

 elevation. 

 Dorsal valve regularly convex in young shells ; old shells more gibbous 

 above, and becoming broadly depressed towards the front, without 

 distinct sinus. Area distinct ; beak scarcely incurved. 

 Surface smooth or marked by concentric striae, which are slightly undu- 

 lated towards the front of the shell ; and in a large specimen, the 

 exfoliated surface is obscurely marked by fine striae. 

 The young specimens of this species have much the aspect of Nucleo- 

 SPIRA ; but when the cardinal line is visible, there is a conspicuous trian- 

 gular fissure with a flattened space, on each side. This feature is 

 sometimes quite marked in the separated ventral valve. The spoon- 

 shaped pit is comparatively wide and deep, and supported on a short 

 septum. In the dorsal valve the lamellae are divergent from their origin. 

 The largest specimen before me has a length of seven-eighths of an 

 inch, with a slightly greater width. 



Geological formation and locality. Collected from strata of Devonian age, at 

 Waterloo, Iowa, by Mr. O. St. John, to whom I am indebted for the specimens. 



