160 PALEONTOLOGY OF THE EUREKA DISTRICT. 



Shell ovoid or subglobose; very inequi valve; hinge line varying from 

 two-thirds to three-fourths of the entire width of the shell. 



Ventral valve arcuate, much deeper than the dorsal, gibbous or regu- 

 larly ventricose above the middle ; beak strongly incurved over that of the 

 opposite valve; fissure of medium size, and bordered by a more or less dis- 

 tinct area, the surface of which is smooth or transversely striated. The 

 spoon-shaped pit in the interior of the valve is moderately extended; 

 median septum very short. Central portion of the valve with a slightly 

 elevated, broad, more or less distinctly defined mesial fold. 



Dorsal valve full above, becoming depressed below the middle and 

 flattened at the sides; the front is flattened and slightly extended, forming 

 a broad, more or less distinct sinus. Lamellae- separate and diverging. 



Surface obscurely plicated on the lower central portion of the larger 

 shells, and one shows traces of very fine radiating striae on the lower part of 

 the ventral valve. 



The specimen described is smaller than the average, having a height 

 and width of 35 mm , with the diameter of the convexity of the valves of 

 SO*""; a large ventral valve is 50 mm in length. 



Formation and localities. Devonian limestone, lower horizon; Comb's 

 Peak, Eureka District, and Lone Mountain, 18 miles northwest of Eureka, 

 Nevada. 



The most striking differences between the Nevada shell and that of 

 Iowa is the great size and the almost entire absence of plications on the 

 former. We cannot, however, avoid the conclusion that they belong to 

 the same species, the Nevada form attaining a greater size and in many 

 instances a broader form. In other examples the specific identity is without 

 question. 



When preparing specimens for the draughtsman it was thought that figs. 

 15, 15 a, &, plate xiv, represented one species, figs. 5, 5 a, b another, and Pen- 

 tamerus occidentalis Hall, loc. cit., still another. 



By the collection of a large series of specimens from the original 

 locality in Nevada, and the bringing together of a fine suit of specimens 

 representing the Devonian P. occidentalis Hall (the latter through the kind- 

 ness of Prof. Samuel Calvin and Rev. William H. Barris, of Iowa), I 



