white.] PALEONTOLOGY — CRETACEOUS FOSSILS. 297 



I am much disposed to agree with the opinion expressed by Mr. Meek, 

 that these two forms are specifically identical. I have no doubt he was 

 influenced in the provisional reference of the shell which he named 

 Cyrena ? holmesi to that genus, rather than to Mactra, by the supposition 

 that the strata from which it was obtained were those of the Lignitic 

 or Laramie Group, which he at that time regarded as Tertiary, because 

 the characters of the shell, so far as he had determined them, were com- 

 mon to both the Cyrenidce and Mactridce. That is, he saw only the lat- 

 eral teeth of the hinge. He mentioned nothing about an external liga- 

 ment, such as characterizes Cyrena, and I have not been able to detect a 

 trace of any such ligament in any of the specimens examined; and 

 those forms from the neighborhood of Colorado Springs certainly do not 

 possess any. 



By a comparison of the figures 4 a and b with 5 a and &, on plate 6, it will 

 be seen that the latter, representing the form from north of Golden City, 

 is shorter than the other form from the neighborhood of Colorado 

 Springs, but not proportionally shorter than Mr. Meek's type ; but, as 

 Mr. Meek has shown, the specimens have all, or nearly all, been dis- 

 torted by pressure in the soft shale in which they are imbedded, and in 

 which they are all in the form of casts. On the contrary, those from 

 near Colorado Springs are completely preserved in form and texture, in- 

 cluding the most delicate markings of the surface. It is possible that 

 these two forms belong to distinct species, but it is not probable. 



There are certain features in these shells, common also to some others 

 that have been discovered in the Cretaceous strata of the West, that sug- 

 gest at least a subgeneric distinction from the typical forms of Mccctra 

 different from any of the published subgenera, but they doubtless belong 

 to the family, Mactridce as before stated. 



Mactra? canonensis Meek. 



Plate 9, figs. 11 a, t, and c. 

 Mactra ? canonensis Meek, 1871, An. Rep. IT. S. Geol. Surv. Terr, for 1870, p. 308. 



The following is Mr. Meek's original description of this species, and 

 the figures of it on plate 9 are from his own drawings : 



" Shell small, very thin, transversely subovate, rather compressed or 

 moderately convex, with length about once and a half the height ; ante- 

 rior side rounded ; posterior side longer, narrower, and obliquely sub- 

 truncated at the extremity; pallial margin forming nearly a semiovate 

 curve, being most prominent anteriorly, straight or very slightly sinuous 

 behind the middle, and rounding up very abruptly to the lower part of 

 the truncated posterior margin; dorsal outline nearly straight and slop- 

 ing to the truncated posterior, and declining more abruptly in front; 

 beaks small, moderately prominent, and placed one-third the length of 

 the valves from the anterior margin ; posterior umbonal slopes rather prom- 

 inent to the posterior basal extremity, while the sides in front of this prom- 

 inence are flattened, or even slightly concave below. Surface with rather 

 regular but distinct lines and furrows of growth. Muscular impressions 

 shallow, posterior round-oval, anterior narrower, with a slender prolon- 

 gation above ; pallia! line with a shallow, rather rounded sinus. 



"Length, 0.78 inch; height, 0.53 inch; convexity, 0.31 inch. 



u Locality and position. — Caiion City [Colo.]." 



It now seems almost certain that this form is specifically identical 

 with J/, holmesi (= Cyrena ? liolmesi Meek), the principal observable dif- 



