FOSSILS OF THE DEVONIAN. 



181 



point, which is above the transverse center of the shell, the outline curves 

 down into the broadly-rounded basal margin; posterior extremity a little 

 below the transverse center and broadly angular, the line above rounding 

 upward into the cardinal line and to the beaks with a uniform curvature; 

 beaks elevated above the hinge-line, rather small, and situated on the ante- 

 rior third of the shell. 



Surface of cast with an obscure anterior muscular scar and obscure 

 concentric lines of growth. 



In the absence of the structure of the hinge the generic reference is ac- 

 companied with more or less uncertainty, but in the light that the present 

 specimens afford the species is referred to the genus Anodontopsis. 



Formation and localities. Lower Devonian horizon of Lone Mountain, 

 18 miles northwest of Eureka; and Grav's Canon, Eureka District, Nevada, 



Genus SCHIZODUS King. 

 CYTHERODON Hall. 



Schizodus (Cytherodon) orbicularis, n.sp. 



Plate v, figs. 8, 8 a. 



Shell of medium size ; suborbicular, or subrhomboidal in outline. Gen- 

 eral surface strongly convex, with the umbonal ridge rounded. Beaks small 

 and but little elevated above the cardinal line. 



Surface smooth or marked by slightly impressed concentric lines of 

 growth. Interior characters unknown. 



Dimensions: height, 3 cm ; breadth, 3.5 cm ; convexity of right valve, .75 cm . 



A small shell (fig. 8 a) still more quadrate in outline occurs in the Lower 

 Devonian of A try pa Peak. It is probable that it is a young shell of the 

 species under consideration. 



In Cytherodon (Schizodus) quadrangular is Hall (Pal. N. Y., vol. v, pt. 1, 

 Plates and Explanations, p. 18, pi. Ixxv, figs. 31-36), from the Chemung and 

 Waverly Groups, we have the nearest form to S. ((7.) orbicularis. 



The beaks in the former are more subcentral and the outline less sub- 

 orbicular. They are, however, very closely allied species. 



Formation and locality. Lower Devonian of Lone Mountain, 18 miles 

 northwest of Eureka, Nevada. 



