16 GEOLOGICAL SURVEY OF THE TERRITORIES. 
tinctly but not very greatly inequivalve; the left valve, as usual, more 
convex than the right, and its beak more prominent than the other; the 
convexity of the surface of the valves somewhat uniform, but increas- 
ing toward the umbonal region, where it is greatest; anterior wing mod- 
erately large, defined from the body of the shell by being laterally com- 
pressed, but not by any distinct auricular furrow; the byssal sinus under 
the anterior wing of the right valve having the usual size and shape 
common to Oxytoma; posterior wing not proportionally large, and not 
distinetly defined from the body of the shell except by a somewhat grad- 
ual lateral compression; its posterior angle not greatly produced ; hinge- 
line less than the axial length of the shell; posterior adductor scars not 
distinct; anterior scars distinct for a shell of this genus, placed imme- 
diately in front of the beaks, that of the left valve being more distinct 
than the other. : 
This, like the following described species, is known only from natural 
casts in brown hematite of the interior of the shell, the imperfection of 
which will not allow of an accurate measurement of all its proportions. 
It is, however, known to have reached an axial length of more than 60 
millimeters, a transverse width near its base of at least 50 millimeters, 
and a thickness of about 25 millimeters, when both valves were in natural 
position. 
The character of the surface is not known, but it was evidently nearly 
smooth, as is usual with its congeners. Itis related to Pteria (Oxytoma) 
nebrascana Evans & Shumard, but it is a larger and more robust shell, 
having a proportionally larger anterior wing, more prominent beaks, 
and broader base. 
Position and locality—Strata of the Dakota Group, Saline County, 
Kansas, where it was discovered by Prof. B. F. Mudge, associated with 
the next following described species, and also with Cyrena dakotaensis 
Meek & Hayden and Cardium kansasense Meek. 
Genus GERVILLIA Defrance. 
GERVILLIA MUDGEANA White. 
Plate 14, figs. 3 a and b. 
Gervillia mudgeana White, May, 1880, Proc. U. S. National Museum, vol. ii, p. 295. 
This shell, like the last described, is known only from natural casts in 
brown hematite of the interior and of a few adhering fragments showing 
the character of the test. It is moderately large, laterally distorted ; 
hinge-line comparatively long, very oblique with the axis of the shell, 
producing a somewhat prominent posterior alation, which is not dis- 
tinctly defined from the body of the shell; cartilage-pits in the area of 
each valve six or seven, as shown by undulations upon the cast; beaks 
placed very near the anterior end, beyond which there appears to have 
been no distinct anterior ear; beak of the right valve more prominent 
than the other, although the right valve is less convex transversely than 
the left; right valve having a somewhat regular and strong longitudinal 
convexity, but its transverse convexity is very slight in the anterior half, 
while that of its posterior half is nearly flat; left valve nearly straight 
or even Slightly concave longitudinally along the axis, but very strongly 
convex transversely in all parts, this convexity being more abrupt along 
the axis than elsewhere, but there is between the axis and the hinge- 
margin a slightly raised rounded fold which extends from behind the 
beak to the posterior margin; adductor muscular impression large and 
