72 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



Vol sell si M «• f k i i, E. & S. (sp.). 



Plate 15, figs. 3, a, h, c. 



Mytilus Meehii, Evans and Shumard (1857), Trans. St. Louis Acad. So., I, 40. 

 Modiola Micl.ii, Meek and llaydeu (I860), Proceed. Acad. N. S. Philad., XII, 427. 



Shell thin, very narrow or depressed-subovate, with length about half 

 the height, moderately convex in the central region, and cuneate behind ; 

 cardinal margin more or less arcuate, and rounding down regularly into the 

 posterior margin, which is rather narrowly rounded below; beaks very oblique, 

 and convex, or rather compressed, and placed over the anterior extremity of 

 the valves; anterior margin very short, compressed, and abruptly rounded; 

 ventral margin nearly straight, or slightly sinuous along the middle; surface 

 with fine concentric .striae ; anterior muscular impression distinct, small, and 

 quite near the anterior basal margin ; posterior very faint, or obsolete. 



Length, 1.39 inches: height, 0.60 inch; convexity, 0.47 inch. 



Young specimens of this species, half an inch and less in length, are 

 narrow-ovate in outline, but, as it advanced in age, it became proportionally 

 longer, so as to assume a narrow oblong-oval, or very narrow subelliptic form. 

 The beaks are placed nearly over the anterior border, which is very narrowly 

 rounded, and extends slightly beyond them. The hinge-line is a little arcuate, 

 and between one-half and one-third the entire length of the shell. It ranges 

 at an angle of about 30° to the axis of the umbones, and passes very gradually 

 behind into the postero-dorsal margin, which is a little prominent and more or 

 less carinate. The base, although generally nearly straight, is sometimes very 

 slightly arcuate in large specimens. The muscular and pallial impressions 

 must be very faintly marked, as scarcely any traces of them can be seen on 

 smooth, well-preserved internal casts. 



Locality and position. — The specimens first described by Drs. Evans and 

 Shumard were obtained on Mpreau River, probably from the Fox Hills group, 

 or formation No. 5. Ours are all from a locality on the Yellowstone, from 

 beds containing a blending of the fossils of both the Fox Hills and the Fort 

 Pierre groups (formations Nos. 4 and 5). It is probably confined to No. 5, 

 where these formations are both distinctly represented. 



