78 GEOLOGICAL SURVEY OF THE TERRITORIES. 
ately and evenly gibbous, the greatest convexity being in the central 
region; anterior margin prominently and rather narrowly rounded ; 
posterior vertically subtruncated; base forming a more or less nearly — 
semi-elliptic or semi-ovate curve; dorsal outline sloping from the beaks, 
the anterior slope being more abrupt and concave in outline, while the 
posterior is generally convex; umbones subcentral, moderately promi- 
nent or somewhat depressed, usually eroded, and more or less flattened 
near the apices which are not strongly incurved, distinctly pointed, or 
raised much above the hinge margin; lunular region in the specimens 
with more gibbous umbones, somewhat excavated, but not distinctly 
impressed, or with defined margins; ligament narrow and not very 
prominent jin some well-preserved examples it is quite ‘:prominent] ; 
anterior muscular impression ovate, well defined, and distinct from the 
small pedal sear under the hinge, above and behind its upper end; 
posterior muscular impression broader and more shallow; pallial line 
usually well defined and provided with a shallow, rounded, or semi- 
circular sinus; hinge rather strong, with the three cardinal teeth well 
developed in each valve; the anterior two of the left valve and the pos- 
terior two of the right being more or less suleated; anterior lateral teeth 
long, linear, and not very prominent; posterior shorter and very remote 
from the cardinals; both anterior and posterior laterals very nearly 
smooth or minutely granulo-striate. 
‘‘ Length of a medium-sized adult specimen, 1.62 inches; height, 1.28 
inches; convexity, 0.92 inch. Some fragments indicate one-third greater 
size for the largest. . 
“Locality and position—Two hundred miles east of Denver City, on 
the Kansas Pacific Railroad, when they were found in a shaft at a depth 
of forty feet below the surface.” 
It also occurs in the Laramie strata of the valleys of Crow and Bijou 
Creeks, Northern Colorado. At the former locality many well-preserved 
examples were obtained, some of which are figured on plate 21. 
CORBICULA (LEPTESTHES) MACROPISTHA White. 
Plate 23, figs. 4 a, b, ¢, d, e, and f, 
Corbicula (Leptesthes) macropistha White, 1878, Bull. U. 8. Geol. Sur. Terr., vol. iv, 
p. 713. 
Shell small, longitudinally subelliptical or subovate, broader (higher) 
posteriorly than anteriorly, slightly gibbous or somewhat compressed 
in the central portion; test strong, but not massive; basal margin 
broadly convex; posterior margin truncating the shell, its direction be- 
ing almost perpendicular or inclining a little backward from below, and 
somewhat abruptly rounded to both the postero-dorsal and basal mar- 
gins; posterior cardinal margin broadly convex; anterior cardinal mar- 
gin nearly straight and directed obliquely downward and forward to the 
front, which is abruptly rounded to the base; beaks depressed, not well 
defined, not projecting above the hinge-margin, and situated about one- 
third the length of the shell from the front. Surface showing the usual 
lines and imbrications of growth; and well-preserved examples show 
that the former were so fine on a large part of the surface as to give it 
an almost polished aspect. Lateral teeth well developed and finely 
crenulate; cardinal teeth having the usual characteristics of the genus; 
pallial line somewhat distant from the margin; sinus shallow. 
Length of an average sized example, as indicated by the twenty or 
thirty specimens in the collection, 21 millimeters; height of the same, 
