FOSSILS OF THE DEVONIAN. 



179 



Hinge-line straight, not quite as long as the greatest width of the shell. 

 Beaks small, situated on the posterior portion of the anterior half of the 

 shell, and nearly but not quite emarginate. 



Surface marked by traces of a few concentric lines of growth. No 

 interior characters have been observed. 



Dimensions: height, 2.25 cm ; breadth, 2.5 cm ; convexity of right valve 

 at the umbo, 2.5 mm . 



In its smooth surface this species varies from typical examples of 

 Posidonomya, but its straight hinge-line and general form ally it more nearly 

 to that genus than to any other with which I am acquainted. 



Formation and locality. Lower Devonian of Atrypa Peak, Eureka Dis- 

 trict, Nevada, also at the same horizon on Lone Mountain, 18 miles north- 

 west of Eureka, Nevada. 



Posidonomya Devonica, n. sp. 

 Plate iv, fig. 7. 



Shell thin, small, obliquely truncato-ovate. General surface very much 

 depressed. Hinge-line straight. (Owing to the breaking away of the ante- 

 rior portion this is not certain, but if any deflection from a straight line 

 occurs it is very slight). Beaks small, situated near the cardinal margin at 

 the anterior third of the distance between the posterior and anterior margins. 



Surface marked by strong concentric lines that terminate on the straight 

 cardinal line. 



Interior and hinge characters unknown. 



Dimensions: height, !4 mm ; breadth, 13 mm ; convexity of right valve, 1.5 1 " 111 . 



This species occurs at the same locality as the preceding, but not in 

 the same layer of rock. It differs in form and surface markings, in the 

 latter feature more closely resembling the typical forms of the genus than 

 P. Icevis. 



As with P. Icevis, we know nothing of the hinge-line or interior char- 

 acters, and have only the general form to go by in making the generic 

 reference, which renders it more or less provisional and doubtful. Com- 

 paring this species, however, with Posidonomya Bronni Voltz, from the 



