520 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



CYRENIDiE. 

 Genus CORBICULA, Muhfeldt (see page 160). 

 Corbie u la c y t lie ri f o rill is, M. & H ■ 



Plate 40, figs. 5, a, b, c, d, e. 



Cyrena (Corttculal) cytheriformis, Meek and Hayden (1860), Proceed. Acad. Nat. Sci. Philad., XII, 176. 

 Corbicitla cytheriformis, Meek aad Hayden (1860), ib., 432. 



Shell attaining a large size, transversely ovate-subtrigonal, or varying to 

 subcircular, but always a little longer than high, moderately convex, rather 

 thick and strong; extremities more or less rounded, or the posterior some- 

 times faintly subtruncated ; basal outline forming a semi-ovate curve, the most 

 prominent part of which is generally in advance of the middle ; dorsal margin 

 sloping unequally from the beaks, the anterior slope being more abrupt and 

 slightly concave in outline, and the posterior convex; beaks moderately 

 prominent, and placed more or less in advance of the middle ; surface only 

 marked with lines of growth, without distinct concentric furrows. 



Length of a medium-sized rather transverse specimen, 1.20 inches; 

 height, 1.05 inches ; convexity, 0.77 inch. 



This species varies more or less in form, some individuals being more 

 transverse, and having the beaks placed farther forward; but it seems im- 

 possible to separate these varieties, on account of intermediate forms. The 

 form represented by figures 5, a, c, plate 40, may be regarded as the typical 

 one. Others, such as those represented by our figures 5, b and d, are the 

 shorter varieties. These latter approach more nearly the following-described 

 species, but are still proportionally more transverse, and have less prominent 

 beaks. 



The specimen represented by our figure 5, e, is a very thick shell, of 

 larger size, that I have sometimes thought might be distinct. It is not in a 

 condition to show its entire outline ; but on following the outlines of its 

 marks of growth, it seems to have agreed nearly in form with the typical 

 specimens of this species when of the same size. 



It is barely possible that this species may not be distinct from the 

 following ; but I have seen no intermediate forms connecting them. 



Like that shell, it more nearly resembles Paris-basin Eocene species 

 than any Wealden or other foreign Cretaceous forms known to me. It is an 

 interesting fact that specimens now before me from Black Butte station 



