GEOLOGICAL SUEVEF OF THE TERRITORIES. 511 



exact in all respects, is to be observed in tlie muscular impressions, 

 there being only one central impression represented in bis Fig. 7 e, 

 instead of three; though above this, he shows the same kind of a little 

 transverse cartilage '? pit, and small round impression by its side, seen 

 in our shell ; only both are placed farther in from the margin. Still, as 

 the larger central scar of his figure shows some slight indications of 

 division, and the three central scars in our shell are sometimes in con- 

 tact, I should not be surprised if a critical comparison would show that 

 there are no real differences between the muscular impressions of the 

 Texas shell and that under consideration. That our shell is not an 

 oyster, however, is clearly evident, from its muscular impressions, as 

 w^ll as from its want of a marginal cartilage-facet, and never having its 

 beaks quite marginal. From the same submargiual character of the 

 beaks in the form figured by Roemer, and its want of a true marginal 

 cartilage-facet, as well as from its general physiognomy, I do not hesi- 

 tate to express the opinion that it is not a true oyster. While looking 

 through the California State geological collection, at San Francisco, 

 last summer, I saw specimens of a shell either identical with that here 

 described, or very closely allied to it, marked "San Diego, Cretaceous, 

 Mr. Morse, collector." 



Locality and i)osition. — Two miles below Point of Kocks, Wj'oming. 

 Bitter Creek series. 



CoRBicuLA (Veloeitina) cythsrifoemis, M. and H.? 



Cyrena (Corbicula?) ajtheriformrs, M. and H., 1860. Proceed. Acad., May, p. 176. 



The shell I here refer with doubt to the above species is certainly a 

 very closely allied form, if not the same. About the only external 

 difference I have been able to see, on comparison with the types of the 

 C. cytheriformis, is that specimens here under consideration usually have 

 the beaks rather more elevated, a little more pointed, and slightly less 

 oblique. Their posterior dorsal slope is also a little straighter in most 

 specimens. On examininga considerable number of individuals, however, 

 they are found to vary more or less in most of these characters, so that 

 if there are no differences in the hinge and interior, the slight external 

 differences may be only such as might be produced by local conditions. 



Of the form under consideration, we have specimens showing perfectly 

 the hinge, and muscular and pallial impressions, as well as all of the 

 external characters. These show that the cardinal teeth have the back- 

 ward obliquity of the type of the group Veloritina, and that the laterals 

 are elongated nearly as in Corbicula, though not quite so long, nor so 

 distinctly striated; and that the posterior lateral is separated, by nearly 

 its own. length, from the posterior cardinal. The hinge characters are, 

 therefore, almost exactly intermediate between Corbicula and Veloritina, 

 the chief difference from the latter group being the slender elongated 

 character of the anterior lateral tooth, as in Corbicula. Unfortunately 

 we have no specimens of the typical C. cytheriformis sbowing the hinge 

 clearly enough for minute comparisons. 1 suspect, however, that specific 

 differences will be found between the hinge of that form and the speci- 

 mens under consideration. Both have the pallial impression provided 

 with a shallow rounded sinus. 



Locality and position. — Point of Eocks, on Bitter Creek, Wyoming, 

 near the bottom of section seen there ; as well as at a much higher hori- 

 zon, at Black Butte Station, on the Union Pacific Eailroad, Wyoming. 

 Bitter Creek series. 



