382 REPORT ON THE STATE CABINET. 



This species of Pentamerus is usually from one inch to an inch and a 

 half in length. It is readily distinguished from other species in the rock 

 by its narrowness at the beaks, and by the numerous longitudinal stria3 

 which mark the casts. 



Formation and Locality. — In limestone of the age of the Niagara group, 

 at Wauwatosa and Waukesha, Wisconsin. 



Pentamerus (Pentamerella) ventricosus. 



PLATE XIII, FIGS. 18-21. 

 Pentamerus ventricosus, Hall. Rep. of Prog. Geol. Survey Wisconsin for 1859, 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 sinu- 

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

 linguiform extension in front. The sinus of the ventral valve and 

 the mesial fold of the dorsal valve are marked by three or four 

 obscure, or sometimes conspicuous, rounded plications. The casts 

 are marked by strong concentric laminae of growth, and some faint, 

 rather broad, radiating stri^: 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, reaching 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 half way to the basal margin. 



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

 merus, and will probably fall under the proposed genus Pentamerella.* 



Formation and Locality. — In limestone of the age of the Niagara group, 

 at Waukesha, Saukville and elsewhere in Wisconsin, and at Bridgeport 1 

 Illinois. 



* See page 193 of this Report. 



