288 EEPOKT UNITED STATES GEOLOGICAL SURVEY. 



Associated with the typical forms of this species, which are represented 

 on plate 5, are others, certainly congeneric with, and perhaps only a 

 variety of this species. It is a more gibbous shell, subcircular, or, at 

 least, more rounded in outline, and with scarcely a trace of the umbonal 

 ridge that marks the typical forms. In these respects it closely resem- 

 bles C. evansi Hall & Meek, already mentioned. 



Position and locality. — Cretaceous strata, probably of the age of the 

 Fox Hills Group: Cimarron, JS^. Mex., where they were collected by 

 Prof. St. John. 



Genus TAE^CEEDIA Lycett. 

 Tanceedia? coelionotus (sp. nov.). 



Plate 5, figs. 2, a, i, c, and d. 



Shell irregularly oblong, transverse, moderately convex ; margins closed 

 all around ; anterior portion narrower and much longer than the posterior ; 

 beaks rather large, elevated, especially so above the antero-dorsal margin, 

 approximate, incurved and directed forward; posterior portion very short, 

 obliquely truncated by a broad and regular curve extending from the 

 beaks to the posterobasal margin, which latter is abruptly rounded 

 to the base; basal margin broadly convex; front regularly rounded, 

 ioining the antero-dorsal margin by a somewhat more abrupt curve than 

 that by which it joins the base; a large heart-shaped lunule deeply im- 

 pressed in front of the beaks but less distinctly defined farther forward; 

 a deep inflexion of the hinge-margin, resembling a deep escutcheon, ex- 

 tends from the beaks to about midheight of the posterior end of the shell, 

 so that the hinge-margin is entirely obscured from a side view ; posterior 

 muscular impression moderately large, oval, situated near the posterior 

 margin and a little below the midheight of the shell ; pallial sinus mod- 

 erately deep, extending nearly horizontally forward ; hinge unknown ; 

 ligament apparently external. Surface marked only by the ordinary 

 lines of growth. 



Length, 37 millimeters ; height from base to umbo, 29 millimeters. 



This species is referred with much doubt to the genus Tancredia, but 

 it is provisionally so referred on account of its external form, the want 

 of knowledge of its hinge, and the want of positive information as to the 

 true character of the pallial line in that genus. Its moderately thick 

 test, closed margins, strong muscular impressions, and pallial sinus sug- 

 gest a close relationship to Donax, and the horizontal position of the 

 sinus is like that of Iphigenia and Hecubaj belonging also to the Bona- 

 cid^e. 



Its broadly rounded front is unlike what we expect to find in Tancredia, 

 and according to some authors its pallial sinus would exclude it from 

 that genus. The characteristic angle of the antero-dorsal margin of 

 MeeMa is also wanting in this species. It is likely that it will hereafter be 

 found to represent an undescribed group of shells, but this cannot now 

 be formally proposed, because no information as to the character of the 

 hinge has been obtained. However, important differences from known 

 genera have here been indicated as existing in this shell, and if a future 

 discovery of correlated differences in the hinge should be made, I pro- 

 pose for it the name of Tancredina. 



Position and locality. — Strata of the Fox Hills Group. The example 

 represented by fig. 2 d, plate 5, was collected by Mr. J. C. Hersey, 10 

 miles west of Greeley, Colo. That represented by figs. 2 o, 6, and c, of the 

 same plate, is from the collection of Prof. Powell's Survey. Its locality 

 is unknown, but it is probably from Northwestern Colorado. 



