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 abruiDtly 

 rounded. 



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

 downward and backward iu a nearly direct course from the dorsal 

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

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

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

 are coarse but indistinct radiating strioe and occasionally still more 

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

 discernible upou 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 confouuded 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. 710V.) — Shell thin, suborbicular iu out- 

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

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

 its outer margin slightly (ioncave; anterior ear moderately large, marked 

 by radiating striae and coiu;entric 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. 



Kadiating stripe 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 i)osteriorly they are even distinctly recurving ; 

 the radiating striae crossed bj' 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 north of 

 Cami) 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 iu 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 ou 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 I F 



