300 GEOLOGY OF THE BLACK HILLS. 



Cardinal area narrow, slightly arcuate and apparently extending to 

 the extremities of the hinge line. Surface of the shell marked by fine, 

 irregular, concentric lines of gi'owth, and also by a few indistinct radiating 

 striai, which are very faint and sometimes obsolete. 



Interior of the hinge marked by two or three slightly oblique lateral 

 teeth, which gradually increase in strength toward the posterior extremity, 

 and on the anterior end by several, more oblique, shorter cardinal teeth. 

 Muscular impressions small and somewhat obscure. 



The shell is decidedly arciform in its general appearance, but the 

 character of the hinge teeth at once separate it from the genus Arca, and 

 the absence of the elevated crest or ridge bordering the muscular impression 

 equally distinguishes it from the genera Cucidlcca Lam. and Idonearca 

 Con. Its relations to the genus Macrodon Lycette, are more intimate, and 

 it is difficult, in the specimen which we have examined, to see where any 

 distinct generic difference exists. 



Formation and locality. — In rocks of Jurassic age, near the Black Hills; 

 exact locality not given. 



MYTILID^. 



Genus MYTILUS Linn. 

 MYTILUS WHITEI. 



Plato 5, figs. 9-12. 

 :.rijtilvs whitei TVliitf., Prelim. Kept. Pal. Black Hills, 1877, p. 18. 



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 mthin 45° with the 

 cardinal line ; posterior margin a little more than half as long as the shell 

 and nearl}- 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. 



