PENTAMERIDiE OF THE HAMILTON GROUP. 
377 
Peiitaiuerella papiBionensis. 
PLATE LVIII. 
Pentamerus papilionensis : Hall, Geol. Rep. Iowa, Vol. i, part ii, cited p. 514. 1858. 
“ *< Idem, Thirteenth Report on the State Cabinet, p. 86. 1860. 
Shell ventricose, broadly ovate, often wider than long, more or less gib¬ 
bous and arcuate in old shells. 
Ventral valve gibbous or ventricose above, becoming depressed in the 
middle into a broad shallow, often undefined sinus, which scarcely 
reaches to the beak and sometimes not much above the middle, and is 
produced in front; sides abruptly curving to the margin : beak 
incurved, obtuse, arching from the broad fissure; cardinal line exten¬ 
ding for more than half the width of the shell. The space above, on 
each side of the fissure, is concave and wrinkled. 
Dorsal valve gibbous in the middle, somewhat regularly curving to the 
sides and front; mesial fold defined below the middle of the valve. 
Surface plicated, the plications rounded or subangular, becoming obso¬ 
lete towards the beak, and. prominent below the middle : of these there 
are two or three in the mesial sinus, and usually about four on the mesial 
fold, with three, four or five on either side. The plications are crossed 
by fine concentric striae of growth, which at irregular intervals are 
crowded into squamose imbricating lines. The entire surface is finely 
papillose or punctate, and when well-preserved might be mistaken 
for a punctate shell. The substance of the shell is lamellose-prismatic 
and brittle. 
The interior of the ventral valve shows a broad short and deep spoon¬ 
shaped pit, the extremity of which isrjeent abruptly to the dorsal side. 
The -septum supporting the conjoined lamellae extends from one-third to 
one-half the length of the valve, and in some examples may extend 
still farther towards the anterior margin. 
The interior of the dorsal valve is not fully known. 
[ Palaeontology IV.] 48 
