FOSSILS OF THE NIAGARA GROUP. 381 



valve most prominent on the umbones ; mesial fold broad and simple 

 on some specimens, and on others divided into two, three or more 

 plications in front ; beak produced and moderately incurved. Ven- 

 tral 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 

 sm-face mai^ked by strong distinct radiating stria3. 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 dichotomizing of the plications 

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

 ventral valve represents very precisely the Racine specimens. 



This species may be readily distinguished from others by the strong 

 angular bifurcating plications, the number of which vary in different 

 individuals ; 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, 

 Racine, Wisconsin. 



GENUS PENTAMERUS, Sowerby. 

 Pentamerus mtjlticostatus, Hall. 



PLATE XIII, FIGS. 22, 23, 24. 



Pentamerus multicostaius. Hall. Rep. Progress Geol. Survey Wisconsin for 1859, 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 strise, 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. 



