VHITE.] JURASSIC FOSSILS. 145 
Genus GERVILLIA Defrance. 
GERVILLIA MONTANAENSIS Meek. 
Plate 37, figs. la and b. 
Gervillia montanaensis Meek, 1873, An. Rep. U. 8. Geol. Sur. Terr. for 1872, p. 472. 
Shell of medium size, distinctly inequivalve, not distorted, the body 
portion rather -lender, generally curved and very oblique with the car- 
dinal border; anterior ear small; posterior ear rather large, flattened, 
and produced at the extremity; cardinal border long and straight; front 
and basal borders forming a continuous and almost regular curve; pos- 
terior extremity abruptly rounded ; postero-auricular border gently 
eurved, with a forward and upward direction of its lower portion, and 
ending with an abrupt backward curve, meeting the cardinal border at 
the extremity of the narrow projection of the posterior ear. Left valve 
more convex than the right, the umbonal portion narrow, and ending in 
an inconspicuous beak ; anterior ear forming a small lobe-like projection 
in front of the umbo, from which it is defined by a faint furrow. Right 
valve corresponding with the left, but it is less convex, and in some 
cases apparently nearly flat. Surface marked by distinct concentric 
lines and some coarser wrinkles of growth; and the median portion of 
the body is also marked by from four to six more or less distinct, slen- 
der, radiating costz, which extend from the umbo to the postero-basal 
border, being separated by spaces of unequal width, but all wider than 
the slender costz. These cost are less distinct upon some examples 
than others, and they are sometimes obsolete or even wanting upon the 
right valve. 
The length of the largest example in the collection was, when perfect, 
about 110" millimeters, measured along its axis, and its hinge line not 
far from 70 millimeters long. 
None of the examples is in a condition to show the character of the 
hinge, and it is, therefore, not certain that this species is a true Gervillia. 
There are upon some of the examples indications of the presence of a 
slender longitudinal ridge near the cardinal border of the posterior ear, 
such as characterizes Pteroperna Morris & Lycett; but this feature is 
obscure. If the hinge was really free from a longer or shorter series of 
cartilage pits, the shell can be referred neither to Gervillia nor Pieroperna, 
but should doubtless be referred to Pteria. In the absence, however, of 
any accurate knowledge of the character of the hinge, I prefer to leave 
it with Gervillia, where it was placed by Mr. Meek. 
Position and locality—Near the lower cation of Yellowstone River, 
Montana, where it was collected by Dr. A. C. Peale, who found it asso- 
ciated with Trig gonia americana, T. montanaensis, Myacites subcdmpressus,, 
and other Jurassic forms. 
Genus VOLSELLA Scopoli. 
VOLSELLA SUBIMBRICATA Meek. 
Plate 37, figs. 2 a, b, and c. 
Modiola (Vulsella) subimbricata Meek, 1873, An. Rep. U. 8. Geol. Sur. Terr. for 1872, p. 
472. 
Shell arcuate-subtrihedral in marginal outline when adult, but more: 
elongate when young; basal margin concave; front margin short, ab-- 
10H 
