PENTAMERID.E OF THE HAMILTON GROUP. 377 



Pentanierella papilionensis. 



PLATE LVIII. 



Pentamerut papiliontntu : Hali, 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 bread 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 strisB 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 is bent abruptly to the dorsal side. 

 The septum supporting the conjoined lamellse 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. 



[ PALiEONTOLOQT IV.] 48 



