white.] PALEONTOLOGY — CRETACEOUS FOSSILS. 299 



eating" the shell obliquely downward and backward, meeting the basal 

 margin by an abrupt curve, and the cardinal margin by an obtuse angle ; 

 cardinal margin nearly straight, not quite parallel with the base, because 

 the shell is wider behind than in front ; front short, truncated downward 

 and forward from the beaks to about midheight of the shell, and then 

 abruptly rounded to the basal margin ; posterior umbonal ridge rounded, 

 prominent, and ending at the projecting postero-basal margin ; postero- 

 dorsal space, or that above the umbonal ridge, so compressed as to make 

 that portion of the shell thin and sharp, quite in contrast with the obtuse 

 angle formed by the meeting of the two valves at the base. Hinge and 

 interior markings unknown. Surface marked by the ordinary lines and 

 imbrications of growth. 



Length, 25 millimeters; height from base to beaks, 8 millimeters; 

 height near the posterior end, 10 millimeters ; greatest thickness, 10 mil- 

 limeters. 



This species is perhaps related to Pachymya truncata Meek,* but differs 

 from that species in having its posterior margin very obliquely, instead 

 of rectangularly, truncated ; its cardinal and basal margins are not par- 

 allel as in that species, and its beaks are placed nearer the front, besides 

 other less conspicuous differences. 



It is referred to Pachymya with some doubt, mainly on account of the 

 thinness of the test and the general delicacy of the shell. It is possible 

 that it shoidd be referred to Trapezium, but no trace of the radiating 

 stripe, which usually mark the surface in that genus, have been detected 

 in this species, and it lacks the general facies as well as the form of that 

 genus. 



Position and locality. — Cretaceous strata of the Fox Hills Group; near 

 the confluence of the Saint Vrains with the South Platte, Northern Colo- 

 rado, where it was collected by Mr. J. C. Hersey, in whose honor the 

 specific name is given. Specimens of this species have also been sent 

 to the office of the Survey by Mr. L. C. Woster, from strata of the same 

 horizon 10 miles west of Greeley, Colo. The Survey is indebted to both 

 these gentlemen for the privilege of examining important and interesting 

 collections of fossils. 



Genus GLTCIMEEIS Lamarck. 

 Glyclmeejs beethoudi (sp. nov.). 



Plate 6, figs. 1 a and h. 



Shell irregularly oblong in marginal outline, moderately gibbous, 

 widely gaping behind; dorsal margin nearly straight, or rising a little 

 backward from the beaks, making it slightly concave ; base straight or 

 slightly emarginate, and subparallel with the dorsal margin ; posterior 

 margin long, obliquely truncating posterior end downward and forward, 

 the basal and posterior margins forming almost a true obtuse angle with 

 each other ; posterior margin abruptly rounded to the dorsal margin ; 

 front regularly rounded, the rounding being a little more abrupt to the 

 dorsal than to the basal margin ; beaks placed a little in advance of the 

 middle, comparatively small, elevated above the hinge-hue, and incurved, 



* In a subsequent publication, Mr. Meek (Annual Eeport U. S. Geol. Sur. for 1872, p. 

 493) expresses the opinion that this species belongs to the genus Trapezium, and not 

 Pachymya. I have not so referred the species here described, although it is possibly- 

 congeneric with that of Mr. Meek, because, while I have doubts as to its being a true 

 Pachymya, I have likewise doubts as to the propriety of referring it to Trapezium, at 

 least before its hinge shall be fully known. 



