72 TRENTON PEKIUD. 



Several varieties of 0. occidentalis have been described as distinct 

 species. Those of the collections agree well with the typical fonns of the 

 species, but it is not unlikely that future examinations of the strata from 

 which they were collected will reveal associated varietal forms similar to 

 those of the typical localities. This shell is one of the most common of 

 those which characterize the rocks of the Cincinnati epoch, ha-sdng been 

 found in strata of that age in Ohio, Indiana, Illinois, Iowa, and Wisconsin, 

 at which places it is understood to prevail in, if it is not confined to, the 

 upper part of the group. 



Position and locality. — Strata of the Trenton period, Cincinnati epoch ; 

 Silver City, Nevada. 



Orthis testudinaria Dalman (?). 



Associated with the foregoing at Silver City, New Mexico, some 



imperfect specimens of an Orthis were obtained that very closely resemble 



0. testudinaria^ but they are not sufficiently perfect to allow of a satisfactory 



determination of their specific identity. Their association in the strata there 



with species that are undoubtedly identical with the common associates of 



0. testudinaria elsewhere adds force to the supposition that our specimens 



belong to that species. 



Orthis plicatella Hall(?). 



Plate IV, fig. 10 a, b, c, and d. 



Orthis plicatella Hall, 1847, paleontology of New York,i, 122. 

 Orthis plicatella Meek, 1873, paleontology of Ohio, i, 108. 



Shell rather small stiboval or somewhat semi-elliptical in outline, wider 

 than long; length of hinge-line generally a little less than the greatest 

 breadth of the shell ; sides regularly rounded to the front, which is broadly 

 rounded with an elliptical curve. 



Dorsal valve not quite so convex as the ventral, most prominent about 

 the middle ; beak depressed ; area directed backward, narrow at the middle, 

 and diminishing to a point at each hinge-extremity; foramen triangular, 

 broad at its base, comparatively large ; muscular scars not distinctly defined, 

 but are separated by a comparatively broad, slightly-raised, medial ridge ; 

 cardinal process small, naiTow, wedge-shaped, the larger end inward, not 

 filling the foramen ; cardinal sockets small ; braclual processes moderately 



