wmTE] PALEONTOLOGY — CRETACEOUS FOSSILS. 299 



catinj? tlie slioll obliquely downward and backward, meeting the basal 

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

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

 the shell is wider behind than ui front ; front short, tnmcated dow^lward 

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

 abru])tly rounded to the basal margin ; posterior umbonal ridge rounded, 

 jn'ominent, and ending- at the projecting postero-basal margin ; i)ostero- 

 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 unknowm. Siuface marked by the ordinary lines and 

 imbrications of growth. 



Length, 25 milluneters; 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 Pacliymya tnmcata 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 dift'erences. 



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 should be referred to Tntpeziiim, but no trace of the radiating 

 strife, 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 Yrains Avith the South Platte, IS'orthern Colo- 

 rado, where it w^as collected by Mr. J. G. Hersey, in whose honor the 

 specific name is given. Specimens of this si)ecies 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 GLTCBIEEIS Lamarck. 

 Gltcimeeis beethgudi (sp. nov.). 



Plate 6, figs. 1 a and 6. 



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 

 shghtly emarginate, and subparallel with the dorsal margin ; posterior 

 margin long, obliquely truncating posterior end downward and forw^ard, 

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

 each other ; posterior margin abrui)tly rounded to the dorsal margin ; 

 front regularly rounded, the romiditig 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-line, and incurved, 



* Li a subsequent piiblication, Mr. Meek (Annual Report U. S. Geol. Sur. for 1872, p. 

 493) expresses the opinion that this species belongs to the genu s Trapezimn, and not 

 I'achiimya. 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 

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

 least before its hinge shall be fully known. 



