FOSSILS OF THE LOWER SILURIAN. 



73 



from the horizon of the Chazy limestone and Calciferous sand-rock of the 

 New York series. This identification and reference gives a greater vertical 

 and geographic range to a species already distinguished for its wide dis- 

 tribution. In England it ranges from the Llandeilo into the Lower Lland- 

 overy, and it is also known in Ireland, Scotland, and Sweden. 



Formation and localities. Pogonip Group, summit of White Mountain, 

 and about 1,000 feet below that horizon on east slope of ridge east of the 

 Hamburg mine, Eureka District, Nevada, and at the same horizon on the 

 north end of Pogonip Ridge, White Pine District, Nevada. 



Orthis Hamburgensis, n. sp. 

 Plate xi, figs. f>, 5 a. 



Shell small, suborbicular in outline, plano-convex; hinge line a little 

 shorter than the greatest breadth of the valves. Dorsal valve moderately 

 convex, most elevated a little behind the center, along a slight ridge formed 

 by two or three strong, slightly-raised striae ; beak small, depressed about 

 half-way down to the cardinal margin. Ventral valve depressed, slightly 

 convex, mesial depression well defined from the beak to the front margin. 



Surface of both valves marked by from 12 to 1 8 strong angular striae, 

 which increase by bifurcation or intercalation towards the margin. 



This species is represented by many separate valves, but none show the 

 area of either valve. It is allied to 0. testudinaria, but differs essentially in 

 the surface ornamentation, which is more like that of 0. plicatella in the fissi- 

 costate character of the stria?. A comparison with the adult shells of 0. pli- 

 catella and 0. fissicosta shows that their surface characters are strongly devel- 

 oped in 0. Hamburgensis, while in the young shells of the same species of 

 the size of the latter these features are not present. 



Formation and localities. Central part of Pogonip Group, on the east 

 slope of the ridge east of the Hamburg ridge; on the east slope of the 

 second ridge southeast of the Jackson mine, and also on the second ridge 

 east of Secret Canon, Eureka District, Nevada. 



