136 GEOLOGICAL SURVEY OF THE TERRITORIES. 
CONCHIFERA. 
Genus NUCULA Lamarck. 
NUCULA PERUMBONATA White. 
Plate 34, figs. 7 a and b. 
Nucula perumbonata White, 1879, Bull. U.S. Geol. Sur. Terr., vol. v, p. 217. 
Shell small, compact, ventricose; posterior aa obliquely truncated 
from the beaks to the postero-basal border, its plane forming an acute 
angle with that border and the sides of the shell; basal border broadly 
convex, front abruptly rounded; umbones large, much elevated ; beaks 
situated about mid-length of the shell, incurved, pointing a little down- 
ward and backward; umbonal ridges forming a right and left angular 
border to the broad, concave postero-dorsal space. ‘ Surface marked by 
fine, raised concentric lines with spaces of like width between them. 
Length, 9 millimeters; height from base to umbo, 64 millimeters; 
thickness, both valves together, 635 millimeters. 
Only one specimen of this species has. been discovered, but it is so 
nearly perfect, and its characteristics are so well marked, and so differ- 
ent from those of any known forms, that I have ventured to give ita 
new specific name. In some of its characteristics this shell resembles 
the young of the next species described, namely, Nuculana obesa, espe- 
cially in the character of the surface lines; but it is not prolonged pos- 
teriorly like Neuculana; besides which it is much more - ventricose than 
the young of Nuculana obesa could have been. It differs from other 
known Carboniferous species of Nucula in the subcentral position of its 
beaks, its large prominent umbones, and the broad concave space 
between its umbonal ridges. 
Position and locality —This, together with most of the following species, 
was collected by Mr. G. K. Gilbert at Wild Band Pockets, Northern 
Arizona, 15 miles southward from Pipe Spring, from Carboniferous strata 
which he regards as inolding a position near the top of the series. 
Genus NUCULANA Link. 
NUCULANA OBESA White. 
Plate 34, figs. 2 a, 6, and ¢. 
. 
Nuculana obesa White, 1879, Bull. U. S. Geol. Sur. Terr., vol. v, p. 216. 
Shell subovate in margmal outline, except that the dorsum 1s concave 
behind the beaks, moderately gibbous in front, but attenuated bebind ; 
basal margin regularly and broadly convex; front regularly rounded, 
the curvature of the margin only slightly lessening between the front 
and the beaks; postero-dorsal margin concave, the posterior portion of 
the shell narrow, and the posterior margin subangular or sharply rounded 
from the postero-dorsal to the postero-basal margin; umbonal ridges 
sharply defined and placed so bigh as to form angular lateral borders to 
the coneave dorsum behind the beaks ; these borders being more sharply 
defined near the beaks than they are further posteriorly ; beaks well 
defined, incurved, and pointed backward, their average distance from 
the front being somewhat more than two-fifths the tull length of the 
