WHITE.] JURASSIC FOSSILS, 151 
cluding the elevated umbones, 23 millimeters; thickness, both valves 
together, 23 millimeters. 
Position and locality.—Jurassic strata near the Lower Caton of the 
Yellowstone river, Montana, where it was collected, together with other 
Jurassic fossils, by Dr. A. C. Peale. 
Genus GONIOMYA Agassiz. 
GONIOMYA MONTANAENSIS Meek. 
Plate 37, fig. 8 a. 
Goniomya montanaensis Meek, 1873, An. Rep. U. 8. Geol. Sur. Terr. for 1872, p. 473. 
The type specimen of this species, which is the only one discovered, is 
imperfect, and its full form is not shown; by means, however, of the fig- 
ure on plate 37, and the following description by Mr. Meek, it may be 
readily recognized: ‘Shell elongate-oblong, moderately convex; ante- 
rior margin regularly rounded; posterior truncated; dorsal and ventral 
margins nearly parallel; beaks depressed and placed near the anterior 
end. Surface having wriukles or cost starting from before the beaks 
and passing obliquely backward and near half way to the base, where 
they die out or become very obscure, and curve horizontally backward 
to meet others passing down to the posterior dorsal slopes.” 
Length, about 47 millimeters; height from base to umbo, about 24 
millimeters. 
Position and locality. Jurassic strata near the Lower Caiion of Yellow- 
stone River, Montana, where it was collected, with other Jurassic fossils, 
by Dr. A. C. Peale. 
Genus MYACITES (Schlotheim) Munster. 
MYACITES SUBCOMPRESSUS Meek. 
Plate 38, figs. 5 a, b, c, d,, and e. 
Myacites (Pleuromya) subcompressa Meek, 1873, An. Rep. U.S. Geol. Sur. Terr. for 1872, 
p. 472. 
Myacites (Pleuromya) subcompressa Meek, 1877, U. S. Geol. Sur. 40th Parallel, vol iv, 
p. 136. 
The collection of specimens of this species, made by parties connected 
with this survey, show it to be a more variable one than would appear 
from Meek’s original full description of it in the second volume above 
cited. A selection of the principal varieties of form have therefore been il- 
lustrated on plate 38, and Mr. Meek’s description is hererepeated. Fig. 
5 ¢is that of a form which may perhaps prove to be a different spe- 
cies, but it is at present regarded as only a variety of M. swbcompressus. 
It is not associated with any of the other specimens of M. subcompressus 
figured on that plate, but comes from Devil’s Slide, Cinnabar Mount- 
ain, Montana. ‘Shell of medium size, oblong-subovate, moderately con- 
vex, the greatest convexity being nearest and above the middle of the 
anterior end; valves nearly closed, or but slightly gaping behind; pos- 
terior margin somewhat abruptly cuneate, rounded in outline, though 
slightly prominent below the middle; basal margin with a moderately 
conyex outline, rounding up more gradually behind than in front; an- 
terior end short, subtruncated; dorsal margin rather short, rounding 
off gradually into the posterior margin; beaks gibbous, but somewhat 
