76 GEOLOGICAL SURVEY OF THE TERRITORIES. 
anterior muscular impression to be ovate, and the posterior, broader, or 
more nearly circular, while the pallial line shows a shallow, rounded 
sinus, forming less than a semi-circle. 
** Locality and position.—Hallville coal mines, just above a bed of coal, 
in a black, argillaceous, rather hard rock, that may be shaly at some 
other places.” 
Figure 2 a on plate 23 represents one of Mr. Meek’s types, from which 
the foregoing description was drawn.* At the time he published that 
description the specimens with a much thicker test were not discovered. 
The latter specimens occur in a different condition of preservation, but 
in equivalent strata, near the top of the Laramie Group, at Black Buttes 
Station, only four miles from Hallville, on the Union Pacific Railroad, 
Wyoming. The species proves to be a much more variable one in form, 
and the natural thickness of the test much greater, than was anticipated 
by Mr. Meek; and there are such good reasons for placing all the Hall- 
ville and Black Buttes forms under one specific name that I do not 
adopt his suggestion of a separate name for any of the forms of this 
type which are found at Black Buttes. Some of the latter forms are 
figured on plates 23 and 21; which figures show a good degree of varia- 
tion, but not the full extent which prevails in the species. — 
The following are Mr. Meek’s remarks on the Black Buttes forms which 
he discusses under the name Corbicula? fracta var. crassiuscula, two 
years after his first description was published ; together with his origi- 
nal remarks on the characteristics of the subgenus Leptesthes, as given 
in the annual report of this survey for 1872, p. 512. They are given at 
length because it is desirable to have the full discussion of this sub- 
genus presented in connection with descriptions and illustrations of all 
its known species. 
“This shell agrees so very closely in form and size, as well as in its hinge 
and pallial and muscular impressions, surface characters, &c., with the 
species I have described from the shale over one of the Hallville beds 
[of coal] under the name Corbicula fracta that it hardly seems proper to 
separate it specifically. Yet in the thickness of the substance of these 
Shells from the two localities and horizons, there is a very marked dif- 
ference, those from Hallville being extremely thin, even in the largest 
specimens, the thickness not measuring more than from 0.02 to 0.03 
inch, while in examples of ‘corresponding size of those here under con- 
sideration it measures from 0.10 to 0.12 inch in thickness. The latter 
also seem to be more convex, but the Hallville specimens, being gener- | 
ally more or less flattened between the lamin of the shale, it is diffi- 
cult to know exactly how far this want of convexity may be due to ac- 
cidental pressure. 
‘“‘T am aware that shells found in argillaceous shales are usually thin- 
ner than examples of the same species from more calcareous deposits ; 
but I have never seen a difference of this kind so strongly marked in 
specimens certainly known to belong to the same species. This thicker 
shell is therefore placed here provisionally as a variety of C. fracta, under 
the name crassiuscula, which it ean retain if further comparisons should 
show it to be specifically distinct. 
— “Yn describing the species C. fracta, I noticed several points of dif- 
ference between it and the characteristic forms of Corbicula and Cyrena, 
and suggested for the group of which it may be regarded as the type, 
the subgeneric name Leptesthes. The peculiarities mentioned were the 
* The flexed outline of the posterior border is regarded as accidental to that speci- 
men only, and not a specific character. 
