126 PALEONTOLOGY OF THE EUREKA DISTRICT. 



Chonetes macrostriata, n. sp. 

 Plate ii, fig. 13; pi. xiii, figs. 14, 14o-c. 



Shell varying in form from tranverse, subelliptical, length and breadth 

 as 2 to 3, to subsemicircular, the length and breadth as 3.5 to 4. Hinge- 

 line a little shorter than the greatest width, in both young and old shells. 

 Cardinal angles obtuse. 



Ventral valve varying from* moderately convex, in young shells, to 

 very convex or gibbose in the older ones. This difference in convexity is 

 also shown in older shells of the same general size, the more narrow indi- 

 viduats being much more convex. The outline of the valve presents the 

 greatest convexity on the umbo, the curvature to the front and lateral mar- 

 gins being regular, while it is more rapidly depressed towards the somewhat 

 flattened cardinal extremities; a feature noticeable in the younger shells 

 and strongly developed in the adults. Dorsal valve more or less conform- 

 able in curvature to the opposite valve. 



Surface marked by strong, rounded striae, the spaces between them 

 being less than their width. They increase by intercalation and bifurca- 

 tion, a shell 15 mm in breadth having twenty on the lower part, and one 40 "^ 

 broad forty, the striae on each having slight undulations in their course. 

 No traces of concentric strise are to be seen. 



The cardinal margin of the ventral valve in the younger shells shows 

 five or six small oblique spines on each side of the apex that in the older 

 shells are not visible in any specimens in the collection. The ventral area 

 is narrow, linear, and has a rather small foramen midway that is partially 

 closed by the cardinal process of the opposite valve. Dorsal area very 

 narrow. 



The imperfect cast of the ventral valve shows a bilobed apex, formed 

 by the diverging dental lamella and narrow median ridge, the muscular 

 impressions being obscured by adhering fragments of the shell. The por- 

 tions of the surface of the interior of this and the dorsal valve that are 

 shown in the specimens are finely papillose, the papillae of the dorsal valve 

 being somewhat coarser than those of the ventral. 



The variation between the young and adult individuals of this species 



