60 GEOLOGICAL SURVEY OF THE TERRITORIES. 
or only very slightly curved, their direction nearly transverse; the pits 
between them in all cases equaling the teeth in width; the teeth which 
occupy the median portion of the hinge, consisting only of numerous 
small but distinct crenulations. Surface marked by ordinary concentric 
lines of growth, and apparently by faint radiating lines, which corre- 
spond in position to the inner crenulations of the pallial border. The 
surface of neither of the only two examples in the collection is well pre- 
served, but it is evidently nearly smooth and not radiately ribbed, as is 
usual in this genus. Upon weathered surfaces radiate lining is dis- 
tinetly seen, but it is probable that it was less distinct upon the natural 
surface. 
Height and width of the largest example, each about 47 millimeters. 
The genus Awinea is regarded as a distinctively marine form, and the 
discovery of this species in the Laramie Group, especially so near its top 
as this form was found, was therefore quite unexpected. Upon its discov- 
ery the idea was first suggested that the specimens had been derived by 
erosion from strata of the pre-existing Fox Hills Group, but the condi- 
tions of their preservation and association were found to be such as to 
leave no doubt that the mollusks which formed them lived in Laramie 
waters. Their immediate associates are Ostrea glabra var. wyomingensis 
and Corbicula occidentalis, all being imbedded in the same layer. These 
two species are there abundant, and the only fresh-water form that was 
found among them was a single example of Viviparus. This is sufficient 
evidence that the waters in which they lived was at least in a good 
degree saline, for the single Viviparus may easily have drifted in among 
the others, while the latter are too numerous to suppose they had drifted 
into fresh waters. Besides which, currents of fresh waters go to saline 
waters, but not the reverse. The inference seems to be necessary, or at 
least natural, that this Axvincwa survived without generic change in the 
freshening waters of the Laramie period, while the marine associates of 
its progenitors failed to survive the changing conditions which wrought 
an almost complete faunal revolution in those waters. There are many 
known cases of survival analogous to this, and, even in this same Lara- 
mie Group, there are two other forms the congeners of which are gen- 
erally regarded as of exclusively marine habitat or affinities, namely Vu- 
culana and Odontobasis. That these forms really survived from former 
marine waters by adaptation to changed conditions, and that their pres- 
ence where we have found them was not due to occasional incursions 
of true marine waters, is apparent from the fact that the localities at 
which they have been found are too distant from the then nearest 
oceanic waters for the latter to have reached them except by extensive 
subsidences of land which evidently did not occur. 
Position and locality.—Only two examples of this shell have been found, 
one by myself, and the other by Mr. W. H. Holmes, in whose honor the 
Specific name is given. They were both obtained at Point of Rocks 
Station, Bitter Creek Valley, Wyoming, from Laramie strata, near the 
top of the group. 
Genus NUCULANA Link. 
NUCULANA INCLARA White. 
Plate 25, fig. 7 a. 
Nuculana inclara White, 1878, Bull. U. 8. Geol Sur. Terr., vol. iv, p. 708. 
_ Shell small, elongate subovate in marginal outline, gradually narrow- 
ing behind the beaks; valves only moderately convex even upon the 
