TO YELLOWSTONE NATIONAL PARK. 143 
Genus MYALINA, De Noninck. 
MYALINA? (GERVILLIA) PERPLANA, 0. sp. 
Plato J, fig. 8. 
Shell rather above a medium size and erect, elongate quadrangular in outline, with a rounded 
basal margin; anterior and posterior borders subparallel, slightly diverging from the cardinal 
margin toward the basal line, which is rather sharply rounded ; height of the shell nearly or twice 
as great as the greatest length in an anterior and posterior direction, and the cardinal border nearly 
two-thirds as long as the greatest length of the shell. Surface of the left valve very depressed- 
convex, the anterior umbonal ridge being low and rounded a little within the anterior margin of 
the shell; beak small, compressed, not projecting beyond the hinge-line. Surface marked by low 
rounded undulations, on the body of the shell, parallel with the lines of growth, which become sharper 
thread-like lines along the postero-cardinal border. 
The depressed and flattened shell, with the subparallel margins aud erect form, will readily serve 
to identily the species. It is possible that the species may prove to be more nearly related to 
the genus Gervillia than to Myalina on the examination of other and better specimens; the surface- 
lining of the shell very closely resembles species of that genus, and the posterior wing is somewhat 
unlike Ayalina, while its erect form is quite unlike Gervillia. 
Formation and locality—In rocks of Jurassic age at Bridger Mountains, Montana ; associated 
with well-known Jurassic fossils. 
Genus VINNA, Linn. 
Pinna LUDLOVI, n. sp, 
Plate 1, figs. 6 and 7. 
Shell elongate-triangular, very gradually increasing in width from the beaks toward the base ; 
the dorsal and byssal inargins diverging at an angle of but little more than twenty degrees. Dor- 
sal margin straight, as long as, or longer than the body of the shell; basal margin, judging from the 
lines of growth, nearly at right angles to the dorsal margin for a short distance, then directed, 
with a rapidly increasing curvature, to the byssal border. Apex and umbones unknown. Surface 
of the valves angularly convex, the left one the most ventricose, and the angularity quite percepti- 
ble. Both valves are marked, except for a narrow space aloug the byssal margin, by numerous, 
very distinct, and somewhat flexuous radiating ribs, strongest in the middle of the shell, and 
decreasing in strength toward each margin; about twenty-two to twenty-four of the ribs may be 
counted across the middle of the sbell on the specimen figured, most of which are marked along 
the middle by a distinctly-depressed line. Concentric lines distinctly marked and often forming 
undulations in crossing the radii. Evidence of minute, scattered, spine-like projections exists upon 
the surface of the radii. Transverse section across the closed valves angularly elliptical; the 
relative diameters about as one and two. 
The strongly-radiated surface and duplicated ribs are features that will readily distinguish this 
from other described species. 
Formation and locality.—In limestones of the Coal Measures, in the cafion of the Musselshell, 
Montana. 
Genus TAPES, Miihlf. 
TAPES MONTANENSIS, D. Sp. 
Plate 2, figs. 1 and 2. 
Shell small, transversely elongate-elliptical, the length being a little more than twice as great 
as the height; valves very depressed-convex ; beaks subcentral, a little nearer the anterior end, 
very depressed and inconspicuous, scarcely rising above the general slope of the cardinal border ; 
extremities sharply rounded, the anterior end broadest; basal margin broadly rounded, but a little 
more arcuate than the cardinal border. Surface of the shell smooth, and presenting the appearance 
of having been polished, with scarcely perceptible lines of growth. 
