25 



strongly-raised carina extending from the break to the posterior margin, 

 defining a prominent longitudinal angle along the median portion of 

 each valve, a little nearer the ventral than the dorsal border ; section 

 rhomboidal, the sides of the rhomb being slightly convex ; posterior 

 margin oblique with the axis of the shell, forming a distinct but obtuse 

 angle with the dorsal margin. The acute angle, which it would form 

 with the ventral margin if it continued its direct course, is abruptly 

 rounded. 



Surface marked by strong, distinct lines of growth, which run obliquely 

 downward and backward in a nearly direct course from the dorsal 

 margin, across the mesial angle, to near the ventral margin, where they 

 are abruptly flexed forward, and blend with the ventral border. Gross- 

 ing the lines of growth upon the surface above the mesial angle, there 

 are coarse but indistinct radiating striae and occasionally still more 

 indistinct traces of similar ones below that angle, all of which are more 

 discernible upon the anterior than upon the posterior part of the shell. 



Some of the largest specimens measure seven and a half centimeters 

 in width along the posterior margin, and they must have been not less 

 than seventeen centimeters in length when entire. 



The large size, proportionally great width, and angular aspect of this 

 shell distinguish it from any other likely to be confounded with it. 



Position and locality. Strata of Cretaceous age, east of Mount Taylor, 

 one mile south of Pajuate, New Mexico. 



Genus CAMPTONECTES Agassiz. 



CAMPTONECTES PLATESSA (sp. nov.) Shell thin, suborbicular in out- 

 line; hinge-line equaling in length about one-balf of the transverse 

 diameter of the valves ; ears sharply defined ; posterior ear short, flat, 

 its outer margin slightly concave; anterior ear moderately large, marked 

 by radiating striae and concentric lines of growth. 



The anterior ear of the right valve separated from the body -portion 

 by a deep, rather narrow, and somewhat angular sinus, which is about 

 one-half as deep as the length of the ear from its outer extremity to the 

 beak. 



Radiating striae fine, distinct, increasing in number so rapidly that 

 the direction of the outer ends of those behind the middle of the shell 

 is transverse, and farther posteriorly they are even distinctly recurving ; 

 the radiating striae crossed by fine, concentric striae, and occasionally by 

 more distinct lines of growth. 



Height and length, each about forty-five millimeters. 



Position and locality. Strata of Cretaceous age, fifty miles jiorth of 

 Camp Apache, and five miles west of Mineral Spring, Arizona. 



Genus INOCERAMUS Sowerby. 



INOCERAMUS DIMIDIUS (sp. nov.) Shell very small, thin, inflated, 

 sometimes much so, obliquely subovate in outline ; valves subequal, 

 the left one being somewhat more capacious than the right; beaks small, 

 prominent, acute, incurving, and pointing a very little forward; hinge- 

 line straight or nearly so, rather short. 



Surface marked by more or less regular and more or less strong 

 folds or undulations, which in some cases exist on only the upper por- 

 tion of the shell, the remainder being smooth or marked only by fine, 

 concentric lines of growth. This cessation of, or irregularity in, the 

 formation of the concentric folds was sometimes connected with con- 

 siderable distortion of the usual symmetry of the shell. 

 3 IF 



