GENUS PENTAMERUS. 373 



prominent on the umbones ; mesial fold broad and simple on some speci- 

 mens, and on others divided into two, three or more plications in front; 

 beak produced and moderately incurved. Ventral valve larger than the 

 dorsal; beak very prominent, but little incurved; area high; deltidium 

 large, height and width nearly equal ; sinus broad and deep, simple or 

 with two or more plications. Entire surface marked by strong distinct 

 radiating stritie. The specimens are casts of the interior and exterior. 

 From the figures and descriptions of Barrande (loc. cit.), I am com- 

 pelled to regard this American species as identical with the Bohemian 

 form. Not only is the general form and dichotomising of the plications 

 similar, but the peculiar elevation and attenuation of the beak of the 

 ventral represents very precisely the Racine specimens. 



This species may be readily distinguished from others by the strong 

 anoular bifurcating plications, the number of which vary in different indi- 

 viduals ; some being nearly simple, having but four or five on each side of 

 the mesial fold, while others have nearly double that number on the front 

 margin. The number of plications on the mesial fold and sinus are subject 

 to the same variation. 



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

 Bacine, Wisconsin. 



GENUS PENTAMERUS (Sowerby). 



PENTAMERUS MULTICOSTATUS. 



PLATE 13 (4), FIGS. 22, 23 & 24. 



Pentamerus multicostatus : Hall, Geological Survey of Wisconsin ; Report of Pro- 

 gress, p. 1. 1860. 



Shell obtusely subcuneate, the valves nearly equally convex below the 

 middle, ventricose on the umbones : sides somewhat straight, and 

 abruptly widening from the beaks ; greatest width in front, nearly equal 

 to the length of the shell. 



Surface marked by numerous, even, rounded, little elevated striae, which 

 are preserved on the lower half of the cast. Both valves are often 

 slightly impressed in the centre below the middle. Longitudinal septa of 

 the dorsal valve reaching more than one-third the length from the beak. 

 The spoon-shaped cavity of the ventral valve is narrow and deep, and 

 the septum reaches more than half way to the base of the shell. 

 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 striae 



which mark the casts. 



Forination and locality. In limestone of the age of the Niagara group, 



at Wauwatosa and at Waukeslia, Wisconsin. 



