18 PALEONTOLOGY OF THE EUREKA DISTEICT. 



the larger is greatly depressed, but there are examples uniting it with the 

 more elevated valve of A. gemma. 



Formation and localities. Cambrian and Silurian. At the summit of the 

 Secret Canon shale in New York, and Secret Canons, and opposite the dump 

 of the Eichmond mine shaft, Ruby Hill ; it also passes up through the shaly 

 beds capping the massive Hamburg limestone belt into the beds of the 

 Pogonip Group, on the ridge opposite the Hamburg mine; at about the 

 same horizon northeast of Adams Hill, and on the ridge southeast of the 

 Jackson mine, Eureka District, Nevada, The species was found in 1882 

 on I^ogonip Ridge, White Pine District, Nevada. 



Genus KUTORGINA Billings. 



Kutorgina Whitfieldi, n. sp. 

 Plate ix, figs. 4, 4 a. 



Shell convex, hinge line straight and a little less than the greatest 

 width of the shell, sides rounding regularly into the slightly convex frontal 

 margin. 



Ventral valve quite convex, elevated along the center to form a flat 

 depressed fold, and sloping quite rapidly from this to the lateral and cardinal 

 margins; median elevation with about five rather faintly-defined, simple 

 plications that reach up to the higher portion of the valve; beak small, a 

 little depressed, and rising above the area; cardinal margins straight and 

 diverging from the beak at an obtuse angle ; character of area unknown. 



Dorsal valve depressed convex, with a rather wide, shallow, median 

 depression, and two short plications on each side of it towards the front, 

 which are obsolete in some of the specimens; the area between the cardinal 

 edges and the elevation of the sides of the mesial depression is depressed and 

 corresponds to the flattened lateral slope of the ventral valve. 



Surface marked by fine, very clearly defined concentric striae subpar- 

 allel to the front and lateral margins. The striae are crowded together into 

 narrow ridges, giving the appearance mentioned by Mr. Billings as occur- 

 ring on the type species of the genus. 



