84 [April, 



from which the above description is made out. The species may be distin- 

 guished from the hist by its much stronger and more regular concentric lines. 

 Locality and position. Same as last. 



CoRBULA? GREGARiA. Shell Very small, somewhat triangular, subglobose ; 

 right valve more ventricose than the left ; beaks nearly central, gibbous, that of 

 the I'ight valve elevated considerably above the left, incurved ; surfiice polished 

 and marked with a few faint, irregular, concentric Avrinkles, indicating stages 

 of growth ; hinge having under the beaks a single prominent tooth in each valve ; 

 anterior muscular attachment indistinct, posterior raised upon a strongly pro- 

 jecting lamina; palleal impressions scarcely sinuous. Length -13 inch ; breadth 

 10 inch; lieight -13 inch. 



It is with much doubt we refer this little shell to the genus Corbvla. In the 

 inequality of its valves, as well as in the character of its hinge, it agrees ex- 

 actly with that genus, but we have seen no species of Corhula having either of 

 the muscular attachments raised upon a projecting plate, as in this shell. In 

 this respect it is more like Cordilla of Deshays, but it appears to want the spoon 

 shaped projection of the hinge of that genus ; and it is the posterior, instead of 

 the anterior muscular attachment that here forms a projecting plate. We sus 

 pect it belongs to an undescribed genus. 



Locality and position. Yellow-stone River, one hundred and fifty miles from 

 mouth, where it is found in vast numbers compacted together in concretions, in 

 the upper part of No. 4 of the series. 



AsTARTE GREGARIA. Shell Small, subtriangular, rounded below and at the 

 extremities, nearly equilateral, valves moderately convex ; beaks much elevated, 

 rather pointed, incurved, approximate, slightly turned forward ; escutcheon 

 narrow lanceolate ; lunule somewhat broadly lanceolate or narrow ovate, 

 not very distinctly impressed ; surface ornamented by small, but distinct con- 

 centric undulations, and much finer lines of growth ; border smooth ; muscular 

 impressions shallow. Length -21 inch ; breadth -14 inch ; height -21 inch. 



Locality and position. Yellow stone River, from a bed probably near the top 

 of No. 4, if not in No. 5 of the series. Abundant. 



NocDLA sciTULA. Shell ovate, rather ventricose, rounded in front, somewhat 

 contracted and narrowly rounded behind ; umboual region gibbous ; beaks 

 prominent, incurved, approximate, located in advance of the middle ; surface 

 marked with distinctly elevated, regular concentric lines, about equal to the 

 spaces between, and stronger on the middle than towards the extremities of the 

 valves ; dorsal border marked by a distinct longitudinal groove behind the 

 beaks. Length -39 inch ; breadth -21 inch ; height -22 inch. 



From N. ventricosa of Hall and Meek, (see vol. 5, new series. Trans. Am. Acad. 

 Arts and Sciences,) to which it bears some resemblance, this may be distin- 

 guished by its beaks being located nearer the anterior end, and by its larger 

 size and comparatively smaller concentric lines. 



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



Ndcdla Evansi. Shell elongate, narrow elliptical, thin, moderately convex ; 

 cardinal margin nearly straight, marked with a distinct longitudinal groove on 

 each valve ; extremities rounded, posterior end compressed and slightly gaping; 

 inferior border forming an elliptical curve, with sometimes a fiiint impression 

 near the middle; beaks very small, not much elevated, located in advance of 

 the centre ; surface polished and marked with faint lines of growth ; muscular 

 impressions indistinct ; teeth of the hinge numerous, closely interlocked. Length 

 72 inch ; breadth -22 inch ; height -32 inch. 



Dedicated to Dr. John Evans, U. S. Geologist of Oregon Territory. 



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



NucuLA EQuiLATERALis. Our Specimens of this shell are only casts. The 

 species may be characterized as follows : subelliptical, rather convex ; extremi- 

 ties narrowly rounded ; beaks central, somewhat elevated ; umbonal region gib- 

 bous; muscular impressions comparatively large, distinct, but not deep; border 



