294 rAL.EONTOLOGY. 



cardinal slope is narrow and depressed, marked only by transverse lines of 

 <;-ro\vtli. The surface of the ridge is marked by irregular transverse lines 

 of nodes. The body of the shell is marked by a series of low, rounded 

 ridges or undulations, which originate in a point at the edge of the flattened 

 umbonal ridge, and extend in the direction of the postero-basal angle of the 

 shell, where they are again bent forward, forming a series of acute angles 

 marking the vimbonal line of the shell, and are directed with a slightly 

 downward tendency to the anterior margin. Along the umbonal ridge of 

 the valve the undulations are broad and rounded, becoming narrow again 

 in their anterior extension. 



The strong undulations of the body of the shell of this species is a 

 distinguishing feature, and one by which it will be readily recognized. It 

 differs greatly in this feature from T. Conradi M. and li. (Pal. Upper Missouri, 

 p. 83, pi. 3, fig. 11) from the Jurassic of the Black Hills, as well as in the 

 subquadrangular form of the shell, that species being of a subtriangular 

 form. 



Formation and locality. — In light-colored, somewhat shaly, limestones of 

 Jurassic age, near Como, Laramie Plains, Wyoming, associated with Pcnia- 

 crinitcs asteriscus M. and H. and other Jurassic species. Collected by 

 Arnold Hague, esq. 



Genus SEPTOCAEDIA n. gen. 



Sliell bivalve, equivalvc, inequilateral, cardiform. Hinge strong; right 

 A'alve Avith a strong, recurAing-, hooked tooth under the beak, and a dec]) 

 cavity below and exterior to it, which is profoundly excavated in the 

 thickened substance of the shell. In the left valve, a large, deep cavity 

 con-esponds to the tooth of the right valve. Lateral teeth obsolete. Liga- 

 ment external, situated in a groove formed by a thickened, overlapping 

 portion of the shell posterior, to the tooth and corresponding cavity. Ante- 

 rior adductor muscular scar very large and deep, separated from the general 

 cavity of tlio shell by a calcareous plate, or septum, extending across the 

 anterior end of the valve on the inner side of the scar, thereby forming a 

 distinct chamber in each valve. Posterior adductor scar much smaller, 

 situated within the pusteriur cardinal margin. Pedal scars not observed. 



