154 PALzEOKTOLOGY. 



angular from the beaks to the posterior basal extremity. Surface only- 

 marked by fine obscure lines of growth. (Hinge and other internal char- 

 acters unknown.) 



Length, 0.45 inch; height, 0.30 inch; convexity, (). 17 inch. Some 

 specimens of apparently the same species are nearly double the size of that 

 from which the above measurements were taken, and some of the smaller 

 ones are proportionally a little shorter. 



As I am unacquainted with the hinge and other internal characters of 

 this little shell, it is only provisionally referred to the genus Mactra. In addi- 

 tion to this, until conchologists can agree in regard to which one of the several 

 generic types included by Linnaeus in that genus is to be regarded as the 

 typical form, it is impossible to know what we ought to call a shell of this 

 type, even where the specimens are in a condition to show clearly all the 

 generic characters. 



It is a smaller and more depressed shell than any of the known Upper 

 Missouri Cretaceous Mactras, and more closely resembles a species described 

 by Dr. Hayden and myself, from the southwestern base of the Black Hills, 

 under the name Tancredia Warrenana, from beds believed, from their strati- 

 graphical position, to belong to the Jurassic series. The typical specimens 

 of the T. Warrenana are merely casts, showing none of the internal char- 

 acters, but have almost exactly the form and general appearance of the 

 genus Tancredia; though they may belong to some other genus. On critical 

 comparison with the species under consideration, the latter is found to diifer 

 in having its beaks slightly more obtuse, its posterior umbonal slopes less 

 angular, and its posterior margin more rounded in outline. 



The specific name is given in honor of S. F. Emmons, Esq., of the United 

 States Greological Survey of the Fortieth Parallel. 



Locality and position. — East Canon Creek, Wasatch Range, Utah, in an 

 ash-colored sandstone, believed to belong to the upper bed of the Cretaceous 

 of that region. 



Mactra (TKiaoNELLA) "2 akenakia, Meek. 



Plate 14, fig. 5. ] 



Shell attaining a medium size, trigonal-subovate, rather compressed; 

 posterior margin rounded, or sometimes faintly subtruncated ; anterior more 



