374 TWENTIETH REPORT ON THE STATE CABINET. 



PENTAMERUS (Pentamerella) YENTRICOSUS. 



PLATE 13 (4), FIGS. 18-21. 



Pentamerus ventricosus : Hall, Geological Survey of "Wisconsin, Report of Progress, 



p. 2. 1860. 



Shell ventricose, subglobose, wider than long : hinge-line extended and 

 abruptly rounded at the extremities. Ventral valve much the more 

 convex, and broadly sinuate below the middle of the shell ; the sinuosity 

 sometimes not reaching the beak, but prolonged into a broad linguiform 

 extension in front. The sinus of thp ventral valve and the mesial fold of 

 the dorsal valve are marked by three or four obscure, or sometimes con- 

 spicuous, rounded plications. 



The casts are marked by strong concentric laminae of growth, and some 

 faint, rather broad, radiating striae : muscular impression of the ventral 

 valve broad, subcordiform, and radiatingly striated. The triangular pit 

 beneath the beak small and shallow, and the median septum short, reach- 

 ing no more than one-fourth the length along the curve of the valve. 

 Dorsal valve having the septa united at the base of junction with the shell, 

 spreading very slightly above, and reaching nearly halfway to the basal 

 margin. 



This shell presents some variation from the strict characters of Pentame- 

 rus, and will probably fall under the proposed genus Pentamerella* 



Formation and localities. In limestone of the age of the Niagara group, 

 at Waukesha, Saukvillle and elsewhere in Wisconsin, and Bridgeport ? 

 Illinois. 



* See page 163 of this Report. 



