136 PAL/EO]STOLOGY. 



ANATINIl)^. 



Genus MYACITES, Auct. 

 Myacites (Pleuromya) subcompressa, Meek. 



Plato 12, figs. (), Go. 



Myacites [rieuromya) sitbcoinpressa, Meek (1S73), Hayden's Ann. Kcp. U. S. Geo). Survey 

 of tbe Territories, 472. 



Shell of medium size, oblong-subovate, moderately convex, the greatest 

 convexity being nearest and above the middle of the anterior end ; valves 

 nearly closed, or but slightly gaping behind ; posterior margin somewhat 

 abruptly cuneate, rounded in outline, though slightly more prominent below 

 the middle ; basal margin with a moderately convex outline, rounding up 

 more gradually behind than in front ; anterior end short, subtruncated ; 

 dorsal margin rather short, rounding off gradually into the posterior margin ; 

 beaks gibbous, but somewhat flattened on the outer side, rather prominent, 

 and located about half-way between the middle and the anterior end ; anterior 

 umbonal slopes prominently rounded, or forming a rounded undefined ridge, 

 which descends nearly vertically from the anterior side of each beak to the 

 anterior basal margin ; the sides behind this ridge being a little flattened, or 

 possibly sometimes slightly concave below. Surface marked by small, 

 rather regular, but not strongly-defined, concentric ridges that become 

 nearly obsolete on the posterior dorsal region and near the front. 



Length, 1.27 inches; height, 92 inch; convexity, 0.66 inch. 



This shell closely resembles some varieties o( Pleuromi/aferrufjinea and 

 P. impressa, Agassiz, but has the anterior end shorter and more truncated, 

 the concentric ridges of less regularity, and the concavity extending from the 

 beaks to the anterior basal margins of the valves either entirely wanting or 

 very feebly marked. In first preparing this I'eport, I had written the above 

 diagnosis of this species, and prepared the figure on plate 12 ; but being in 

 doubt respecting its affinities, I did not then propose a name for it. Subse- 

 quently numerous specimens of this shell were brought in by Dr. Hayden's 

 party from the Canon of Yellowstone Kiver; Spring Canon; near Fort Hall, 

 .Montana ; and other localities. These show that it varies considerably in 



