1G6 REPORT UNITED STATES GEOLOGICAL SURVEY. 



Xo. 1, second series, p. 43, "from a sliaft sunk on the Kansas Pacific 

 Railroad, 20Q miles east of Denver, Colo., 45 leet below the surface, from 

 beds of the age of the Wyoming Bitter Creek coal series." Besides my 

 own collections of this species already recorded, I found it also abundant 

 in the valley of Crow Creek, where it seemed to be confined to No. 4 of 

 the section there. Upper valves only were discovered, almost all of 

 which plainly show the characteristic radiating strise upon the surface, 

 but upon a few they are obsolete. In view of the apparent identity of 

 the muscular markings and other generic characteristics of Anomia pos- 

 sessed by these shells, it seems imperative that we should regard them 

 as bivalves, and yet it is difficult to understand why no trace of an under 

 valve, among the thousands of upper valves that have been collected, has 

 yet been discovered. These remarks apply with equal force to Anomia 

 gryphorhynchus, also of the Laramie Group, and so far as I am aware, 

 to all the species of that genus in the American Mesozoic strata. It is 

 worthy of remark: that upon some of the shells of the Ostrea found in 

 the neighborhood of Maynard's ranch, already mentioned on previous 

 pages, shells of Anomia micronema were found adhering after the manner 

 of Patella or Crepidula. In some cases they were found adhering to both 

 sides of the oyster-shell, always conforming to the inequalities of the 

 surface of the latter, and in all cases with the interior surface of the 

 Anomia against the oyster-shell ; never the reverse. Furthermore, a 

 careful removal of the adhering shell revealed no trace of an opposite or 

 fellow valve beneath it. These circumstances, together with the fact of 

 the non-discovery of the under valve, as before stated, seem to suggest 

 at least the possibility that only one valve pertained to this mollusk. 

 The existence of the characteristic muscular impressions of Anomia in 

 these shells, implying the necessity for both proximal and distal insertion 

 of the muscles into shelly substance, seems, however, to be decidedly 

 against such a supposition. 



While the bulk of the shell substance of this species, like that of other 

 species of Anomia, is pearly, and often brilliantly so, that portion which 

 is occupied by the nearly centrally located broad muscular scar has a 

 subprismatic structure similar to that of the interlamellar layers upon 

 the outer surface of Ostrea or the subepidermal layers of TJnio, but it is 

 usually less distinctly prismatic than are the portions of the other shells 

 referred to. 



The direction of these shell-fibers in the case of the Anomia being, as 

 in all other cases, perpendicular to the plane of the valve, and yielding 

 to destructive disintegration more readily than the remainder of the 

 shell, it not unfrequently happens that a hole is thus made through the 

 valve, suggesting that it may be the byssal aperture of an under valve. 

 But in ad these cases the muscular markings and other interior char- 

 acteristics show them to be upper valves, and of course without a byssal 

 aperture. 



Anomia micronema, as will be seen from records of species and locali- 

 ties on following pages, is one of the most common species of the Lara- 

 mie Group on both sides of the Bocky Mountains, having also a great 

 vertical range in that group. 



IsTo. 2. Anomia grypJiorliyncJius Meek. 



This species was first described by Meek in the Annual Beport of the 

 United States Geological Survey of the Territories for 1872, p. 509. 

 His type-specimens were very numerous, and came from a stratum in 

 the Bitter Creek series two miles west of Point of Bocks Station on the 

 Union Pacific Railroad, Wyoming, which stratum holds a position sev- 



