IXYKHTEBRATE PALEONTOLOGY. 527 



iv, fig. 13), from tiic Fresh-water Tertiary beds of Northern Bohemia. 

 A careful comparison, however, of specimens, would doubtless show constanl 

 differences, since the S. corneum is not known among our existing American 

 species. lis nearest representative among the existing species of 1 1ds country 

 is probably S. rhomboideum, Say (— Cyclas elegans, Adams), at one time 

 supposed to be a variety of S. corneum. Our species has, however, more 

 tumid beaks than that shell, and never has its valves so flattened or compressed 

 in the middle. 



Locality and position. — Ten miles below Fort Union ; Fort Union group : 

 probably Lower Eocene. 



& p h ae r i u in subellipticum, M. & H. 



Plate 43, figs. 5, a. b. 



Ci/clas snbelliptica, Meek and Hayden (185G), Proceed. Acad. Nat. Sci. Philad., 115. 

 Splicurium subellipticum, Meek and Hayden (1860), ib., 185. 



Shell small, transversely elliptic-subovate, rather convex, thin and fragile; 

 extremities rounded, the posterior end being narrower than the anterior, 

 which seems sometimes to be a little truncated ; base semi-ovate, being more 

 prominent in the central and anterior regions than behind ; cardinal margins 

 nearly straight in the middle, and rounding gradually toward the extremities; 

 beaks rather depressed, but rising above the hinge, moderately convex, 

 scarcely oblique, and located slightly in advance of the middle ; surface 

 marked by fine, rather obscure, concentric striae. 



Length, 0.24 inch ; height, 0.14 inch ; convexity, 0.13 inch. 



This form differs from the last in being more transverse, with a more 

 ovate outline, and a narrower and more rounded posterior margin, as well 

 as less elevated beaks. 



Locality and position. — Same as last. 



s ]» h :<• i i 1 1 hi rccticar din ale, M. & H. 



Plate 4:5, figs. 3, a, b. 

 Sphceriumrecticardinale, Meek and Hayden (I860), Proceed. Acad. Nat. Sci. Philad., VIII, 176. 



Shell of medium size, transversely subelliptical, rather compressed, 

 very thin; anterior side rounded; base forming a regular semi-elliptic curve; 

 posterior extremity obliquely subtruncated above, and rather narrowly rounded 

 below; hinge-margin long and straight; beaks very small, and projecting 



