1^4 PALEONTOLOGY. 



teriorly; anterior margin regularly rounded; base semi-ovate in outline; pos- 

 terior margin somewhat narrowly rounded below and sloping forward above; 

 dorsal margin short ; beaks placed in advance of the middle, a little com- 

 pressed, and directed obliquely forward and inward ; surface marked by 

 concentric striae and furrows ; lateral teeth stout. 



Length, 0.98 inch; height, about 0.92 inch; convexity, about 0.54 inch. 



The specimens of this shell are not in a very good state of preserva- 

 tion, being, with one exception, internal casts, arid this one only retains a 

 part of the shell. They certainly differ, however, from the last not only in 

 their much larger size, but in being less nearly equilateral, more produced, 

 and rather more narrowly rounded posteriorly, as well as proportionally less 

 convex. The internal casts have the umbonal region from a little above 

 the middle of the valves compressed. Some of these casts show a few 

 rather distinct, broad, irregular, concentric undulations that were doubtless 

 more strongly defined on the exterior of the valves. 



None of the specimens of this shell show the hinge very clearly; but, 

 from its large size and thickness, I was at first inclined to believe it a Cyrena 

 or a Corhkula. Impressions in the matrix, however, show that its lateral 

 teeth are not striated, nor of the form seen in the latter genus. Possibly I 

 should call it Cyrena Idahoensis; as its pallial line is certainly simple, how- 

 ever, and not sinuous, as in most of the American living species, and, so 

 far as known, in nearly all the fossil Cyrenas and Corbiculas of this con- 

 tinent, I have concluded to place it provisionally in the genus Sphcerium 

 until better specimens can be obtained for study. 



Locality and position. — Same as last, and from same formation at Castle 

 Creek, Idaho. 



UNIONID^. 



Genus UNIO, Retzius. 

 Unio Haydeni, Meek. 



, Plate 16, figs. 3, 3 a, and 3 b. 

 Unio Haydeni, Meek (ISGO), Proceed. Acad. Nat. Sci. Philad., XII, 312; and (1876) in 

 Col. Simp.son\s Report Expl. across tbe Great Basin of Utah, 364, pi. v, figs. 

 11 a, b. 



Shell rather small, or of nearly medium size, elliptic-subovate, rather 



