PENTAMERIDiE OF THE HAMILTON GROUP. 
379 
Pentamerella ©bsolescens (n. s.). 
PLATE LYIII. 
Shell small, gibbous or ventricose, about as wide as long. 
Ventral valve ventricose above the 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 LYIII. 
Spirifer dubius : Hall, Thirteenth Report oh 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 lamellae. 
Geological formation and locality. This species occurs in rocks of the age of 
the Hamilton group near Iowa city Iowa. 
