FOSSILS OF THE DEVONIAN. 1 27 



is so great that the two extremes will readily be taken as distinct species, 

 to avoid which an illustration is given of the younger and older shells with 

 an intermediate form. The outer surface of the ventral valve of the shell 

 is exfoliated, the characteristic striae showing on the dorsal valve of the 

 same specimens. 



Formation and localities. Lower horizon of the Devonian limestone, 

 Comb's Peak; south spur of Atrypa Peak; on the divide of the Reese and 

 Berry Canon, and midway of Gray's Canon, Eureka District, Nevada; also 

 at the same horizon at the north end of the Ravens Nest, Pinon Range, 



Nevada. 



* 



Choiietes filistriata, n. sp. 

 Plate xiii, figs. 15, 15 a. 



Shell transverse, broadly subelliptical, varying in proportion from 4 to 

 3, to 3 to 2, in breadth and length, respectively; the hinge-line is a trifle 

 shorter than the greatest width of the shell, and the cardinal angles are ex- 

 tended in very short auriculate points. 



Ventral valve moderately convex, the convexity varying but slightly 

 between the young and old shells; the outline of the valve is regularly 

 convex, rising from the cardinal margin and sloping more rapidly from the 

 broad central portion towards the front and lateral margins ; cardinal margin 

 with three or more short, slightly oblique spines on each side of the apex. 

 The area is narrow, linear, and divided midway by a rather broad triangular 

 foramen that is nearly filled by the cardinal process of the opposite valve. 



The dorsal valve is moderately concave, following very nearly the 

 curvature of the ventral valve. Area linear and very narrow. 



Surface marked by numerous fine, closely arranged striae, that increase 

 by intercalation and bifurcation; they extend to nearly the hinge line on 

 the cardinal extremities, and vary in number from 80 to 90 on the lower 

 portion of a medium-sized shell. A few specimens show faint undulating 

 concentric striae. 



The specimens are all preserved in a shaly limestone and the interiors 

 of the valves have not been observed. 



Numerous shells corresponding in general form and size with this spe- 



