142 RECONNAISSANCE FROM CARROLL, MONTANA, 
Facial sutures directed forward on a line with the eye for a short distance, when they are 
directed inward with a strong curvature to the anterior furrow, in front of which they converge 
more rapidly, and, meeting in the median line, give an angular form to the frontal limb when the 
movable cheeks are absent. Movable cheeks subtriangular, exclusive of the posterior spine; cen- 
tral area convex; marginal rim strongly rounded and gradually widening from the front, posteriorly 
to the origin of the spine, which is of moderate strength, and as long as the glabella and frontal 
limb of the head. Surface of the movable cheeks covered with strong granules. The glabella and 
fixed cheeks have been similarly marked, judging from the pustulose surface of the cast of these 
parts. Thorax and pygidium unknown. The surface-structure, together with the well-marked pits 
in the frontal furrow, will serve to distinguish this from any other known species. 
Formation and locality—tIn limestones of the Potsdam Group; at Moss Agate Springs near 
Camp Baker, Montana. 
Genus GRYPH AGA, Lam. 
GRYPH HA PLANOCONVEXA, 0. Sp. 
Plate 2, figs. 9 & 10. 
Shell of medium size; general outline more or less orbicular, or with a straightened cardina 
margin ; transverse section planoconvex. Lower valve more or less rounded, often quite ventricose, 
but sometimes depressed-convex; beak small and narrow-pointed or truncate, usually somewhat 
twisted, projecting slightly beyond the line of the hinge, and often incurved close to the cardina} 
border. Upper valve flat or slightly concave, smaller than the other. Ligamental area of the 
lower valve small; cartilage-groove narrow. Muscular imprints reniform, eccentric. Substance 
of the shell rather thin and nacreous; surface roughly lamellose. 
The form of the shell as seen in several individuals strongly resembles that of an Anomia; but 
on splitting open one of the specimens, it revealed the features of a Gryphea. The general form 
and characters are so distinct from any known species from rocks of Jurassic age that it may be 
readily recognized. 
Formation and locality.—In rocks of Jurassic age in the Bridger Mountains, Montana; asso- 
ciated with Camptonectes bellitriata. C. extenuatus, Gervillia erecta M. & H., and Pleuromya subcom- 
pressa—= Myacites (Pleuromya) subcompressa Meek. 
Genus GERVILLIA, Defrance. 
GERVILLIA SPARSALIRATA, nN. Sp. 
Plate 2, fig. 8. 
Shell small, much below a medium size, very oblique, and rather slender; the axis of the body 
of the shell forming an angle of not more than twenty to twenty-two degrees with the car- 
dinal line. Anterior wing not determined, but apparently very small or obsolete; posterior 
wing proportionally long and narrow, the surface flattened and -the outer angle very obtuse; body 
of the shell convex, the left valve much the most rotund, scarcely or not at all curved in its direc- 
tion. Anterior margin slightly convex; basal margin rounded. Surface of the left valve marked 
by about five comparatively strong radii, with wider interspaces, those along the middle of the 
valve strongest and most distant; also by well marked, crowded, concentric lines, which are more 
distinct in crossing the radii than between, and on the posterior wing are directed toward the hinge 
in an almost direct line. Right valve less convex than the left and the markings less distinet. 
This species somewhat closely resembles G. montanensis Meek (Geol. Surv. of the Territ’s, 1872, 
p. 472), but differs very materially in the smaller angle formed by the body of the shell with the 
hinge-line, and also in the greater length of the hinge. It is possible it may be only a strongly 
marked variety of that species, but this can only be satisfactorily determined by more and better 
specimens. At present, however, it seems impossible to identify it with that one. 
Formation and locality—In rock of Jurassic age at Bridger Mountains, Montana; associated 
with characteristic fossils of that formation. 
