CRETACEOUS FOSSILS. 155 



narrowly rounded, most prominent a little below the middle ; base forming 

 a nearly semi-elliptic cm-ve, being most prominent along the central region, 

 and rounding rather more abruptly into the posterior margin ; dorsal mar- 

 gin sloping from the beaks toward the extremities ; beaks moderately promi- 

 nent, but very slightly oblique, and very nearly central, or placed slightly 

 behind the middle ; posterior umbonal slopes with each a shallow but dis- 

 tinct sulcus extending obliquely from the beak toward the posterior basal 

 margin. Surface ornamented by very regular, distinctly-defined, concentric 

 lines and furrows. (Hinge and interior unknown.) 



Length, 1.48 inches; height, 1.10 inches; convexity, about O.GO inch. 



As the specimens of this shell yet obtained show neither its hinge nor 

 internal characters, it is not possible to determine from them whether it is a 

 true Mactra {Trigonella), or a Spisula, or whether it belongs to some of the other 

 allied groups. It has the form and general external appearance of Jfac^m 

 {Trigonella), but differs from all of the otherwise similar described species 

 of that genus known to me, in the distinctness and regularity of its concen- 

 tric hnes and furrows, as well as in the possession of the oblique posterior 

 umbonal sulcus. In the later character, it seems to agree very nearly 

 with Mactraf tenuistria, Gabb (California Palaeontology, vol. II, plate 29, 

 fig. 68). It dififers, however, in being proportionally longer and more 

 abruptly or narowly rounded in front, much more coarsely striated, and 

 in having the posterior umbonal sulcus of each valve directed so as to reach 

 the posterior margin farther up. This sulcus seems not to impart any dis- 

 tinct sinuosity to the posterior margin, but perhaps gives it a very slightly 

 truncated appearance in some specimens. 



Compared with the last, this species will be seen to diflter, not only in 

 its larger, more gibbous, and less depressed form, but in the possession of 

 proportionally stronger concentric ridges and furrows. 



Locality and position. — Whitish Cretaceous sandstone, including coal, 

 on Red Creek, Uinta Mountains; and in the same rock on Chalk Creek, 

 Utah. 



Mactra (Cymbophora) ? Utahensis, Meek. 



Pl.ate 15, figs. 9, 9 a, and 9 b. 



Shellsubovate, moderately convex; anterior margin rounded; posterior 



