18 



FAM. MYTILDXffi. 



GENUS MYTILUS, Li An. 



MYTILUS WHITEI, n. sp. 



Plate 5, figs. 9-12. 



Shell of moderate size, trapezoidal in. outline, very oblique; hinge 

 line slightly arcuate, as long or a little longer than the distance from 

 the apex to the postero-basal extremity ; anterior or byssal border long, 

 distinctly concave from the apex of the beak to the junction of the basal 

 border, the general direction being at an angle of about 45, or within 

 45 with the cardinal line ; posterior margin a little more than half as 

 long as the shell and nearly parallel to the anterior border, or slightly 

 converging toward the base ; basal margin short and more or less 

 rounded, somewhat variable in different individuals. Beaks small, 

 pointed, terminal and incurved, with a short but distinct area-like 

 space beneath them; umbones prominent, subangular, and with a 

 slightly flattened or concave space on the anterior side, just below the 

 apex of the beak. Umbonal ridge elevated, strongly arcuate and sub- 

 angular, the -surface of the shell sloping rapidly to the posterior and 

 postero-cardinal margins. Anterior slope of the shell nearly or quite 

 vertical, or sometimes a little concave near the middle of the length. 



Surface of the shell marked by numerous strong but very irregular 

 varices of growth, which give to the shell an extremely rugose appear- 

 ance. Bet ween the varices the surface is marked by fine lines of growth. 

 No radiating lines or other surface markings, beyond those described, 

 can be detected. Substance of the shell thick and strong. 



The shell is a strongly marked and very characteristic species, and 

 judging from the fragments in the hand specimens of rock, must have 

 been an abundant form in a certain layer at the locality where collected. 

 We know of no American species of the genus having any close resem- 

 blance to this one. 



Formation and locality. In rocks of Jurassic age, associated with 

 other Jurassic fossils, near Sun-Dance Hills, Northwestern Black Hills, 

 Wyoming. 



GENUS TRAPEZIUM, Humph. (= CYPRICARDIA, Lam.}. 

 TRAPEZIUM BELLEFOURCHENSIS, n. sp . 



Plate 5, figs. 1-4. 



Shell small, transversely elongate-quadrangular, the length being 

 from two and a half to three times greater than the height ; dorsal and 

 basal margin subparallel, the former very slightly arched and the latter 

 scarcely concave, except in the older individuals, the smaller and me- 



