1856.] 



83 



Tellisa subelliptica. Shell small, ovate or elliptical, thin, mnch compressed ; 

 anterior extremity rounded; posterior end somewhat rounded or subtruncate ; 

 beaks small, not much elevated, a little in advance of the centre; surface' rather 

 faintly marked with fine lines of growth. Length -50 inch; breadth -12 inch ; 

 height -32 inch. 



This species is chiefly distinguished from the last by its more broadly rounded 

 extremities, less distinct lines of growth, and greater thickness. 



Locality and position. Cherry Creek, Upper part of No. 5 of the series. 



Tellina PttOUTt. Shell ovate, much compressed, very thin and fragile ; ex- 

 tremities rounded : posterior cnu somewhat contracted, and having a broad, 

 indistinct ridge passing from the beaks obliquely backwards to the postero- 

 inferior margin, the outline of which it may have slightly modified ; cardinal 

 border convex before, and concave behind the beaks ; umbones nearly central, 

 elevated, somewhat gibbous, closely approximate ; surface ornamented with fine 

 but distinct lines of growth ; internal laminaj of the shell marked with faint, 

 very fine radiating lines ; sinus of the palleal impression oblong, obtuse or sub- 

 truncate at the extremity, about one-third the length of the shell. Length 2*42 

 inches; breadth -83 inch ; height 1-GO inches. 



We have not seen the hinge of this shell. The species is dedicated to Dr. H. 

 A. Prout, of St. Louis, Missouri, to whom we are indebted for the only speci- 

 men we have seen. 



Locality and position. Fort Benton, No. 4 of the series, 



Cytherea Deweyi. Shell subcircular or slightly oval, somewhat compressed; 

 beaks moderately elevated, a little in advance of the centre ; surface ornamented 

 by distinct irregular lines of growth ; lunule small, narrow, oval or broad lan- 

 ceolate, not very distinctly impressed ; muscular impressions shallow, anterior 

 one narrow ovate ; posterior broad ovate, acutely angular above ; palleal im- 

 pression having a rather deep triangular sinus, forming at the ajiex an angle of 

 55; border smooth. Length -96 inch ; breadth -51 inch ; height "85 inch. 



Specimens slightly more elongated, but apparently identical with this, occur 

 on the Yellow-stone River in a bed we have considered the upper part of No. 4, 

 but which may represent No. 5. This species is dedicated to Prof. Chester 

 Dewey, of Rochester University, N. Y. 



Locality and position. Moreau River, No. 5 of the series. 



Cytherea Nebrascensis. Shell subcircular, much compressed ; beaks somewhat 

 elevated, small, a little in advance of the centre ; ligamentary cavity long and 

 very narrow lanceolate ; surface marked with rather faint lines of growth. Length 

 VO inch ; breadth -28 inch ; height -GO inch. 



The much more compressed form of this shell will serve to distinguish it at 

 once from the last. 



Locality and position. Same as preceding. 



CoRBULA VE?iTRicosA. Shell Small, very thin, subglobose, nearly equivalve ; 

 anterior end broadly rounded ; posterior extremity abruptly contracted into a 

 narrow prolongation ; beaks elevated, ventricose, a little behind the centre of 

 the globose part of the shell; surface marked with distinct irregular lines of 

 growth ; cardinal tooth of right valve prominent, flattened ; posterior muscular 

 impression deep ; palleal impression having a broad triangular sinus. Length 

 55 inch ; breadth -26 inch ; height -27 inch. 



Locality and position. Same as last. 



CoRBULA MoREAUENSis. Shell Small, subovate, ventricose, rounded in front, 

 suddenly contracted and somewhat attenuate behind ; beaks prominent, situated 

 behind the middle of the broadest part of the shell, rather inclined backwards ; 

 surface ornamented with strong, regular, elevated concentric lines, nearly equal- 

 ling the spaces between. Length about -27 inch ; breadth -10 inch ; height '16 

 inch. 



We have not yet seen the left valve of this shell, nor the interior of the right, 



