PENTAMERIDiE OF THE HAMILTON aROUP. 379 



Pentamerella obsolescens (n. s.). 



PLATE LVIII. 



Shell small, gibbous or ventricose, about as wide as long. 



Ventral valve ventricose above tbe middle ; umbo prominent : beak 

 abruptly incurved over a large triangular fissure, with the shell flat- 

 tened along the cardinal margin. 



Dorsal valve regularly convex below, gibbous above : beak prominent, 

 scarcely incurved, with an apparent narrow area on each side the 

 fissure. 



Surface smooth, or with nearly obsolete radiating folds. 



Interior of ventral valve with a pit or trough supported on a short 



septum ; and in the dorsal valve there is a short sessile trough made by 



the converging lamellae, which reaches to about the middle of the length 



of the valve. 



Geological formation and locality. In shale of Devonian age : Waterloo, Iowa. 



Received from Mr. O. St. John. 



Pentamerella dubia. 



PLATE LVIII. 

 Spiri/er dubiut : Hall, Thirteenth Report on the State Cabinet, p. 90. 1860. 



Shell ovoid, ventricose ; cardinal extremities rounded ; length and width 

 sometimes equal, the length in young shells greater than the width : 

 depth of the two valves equal to two-thirds the width. 



Ventral valve gibbous or ventricose, extremely arcuate, with a flattening 

 or subsinuation and extension of the front : umbo prominent, and 

 extended beyond that of the opposite valve one-fifth or more of its 

 length ; apex strongly incurved. 



Dorsal valve gibbous, much shorter than the ventral, prominent in the 

 middle above, with a more or less defined mesial fold towards the 

 anterior margin, which is marked by four or five plications which 

 coalesce and become obsolete before reaching the apex. 



Surface marked by from fourteen to twenty or more plications which 

 show a tendency to bifurcate near the margin, and in old shells are 

 entirely obsolete on the upper part of the valves : plications crossed 

 by imbricating lamelloD. 

 Geological formation and locality. This species occurs in rocks of the age of 



the Hamilton group near Iowa city, Iowa. 



