18 PALEONTOLOGY OF THE EUREKA DISTRICT. 
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 Cation shale in New York, and Secret Camions, and opposite the dump 
of the Richmond 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 Pogonip Ridge, White Pine District, Nevada. 
Genus KUTORGINA Billings. 
Kutorgina Whitfieldi, n. sp. 
Plate ix, figs. 4, 4a. 
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 striee subpar- 
allel to the front and lateral margins. The strize are crowded together into 
narrow ridges, giving the appearance mentioned by Mr. Billings as occur- 
ring on the type species of the genus. 
