WHITE.] JURASSIC FOSSILS. 147 
paleozoic to mesozoic strata, but such a difference in age is strong pre- 
sumptive evidence that there is also a difference of generic characters 
between shells from the strata of each respectively, even though they 
may agree in external form. Therefore, in absence of any knowledge 
of the hinge of the species here described, I think it unadvisable to refer 
it to Modiomorpha ; and thinking the suggestion of Mr. Meek a correct 
one, that these shells probably represent a new genus, for which he pro- 
posed the name Modiolina, I have here used that name in parenthesis 
as a provisional generic reference. 
Position and locality.—This shell was discovered by Prof. O. St. John, 
associated with Aviculopecten superstrictus and afew other obscure forms, 
at the head of Lincoln Valley, Southeastern Idaho. Although both this 
species and A. superstrictus are here referred to strata of Jurassic age, 
it is, as has already been stated, not certain that they are not of Trias- 
sic age, especially since both species have close congeners in the strata 
of the latter period in the same region from which these were obtained. 
Genus MYTILUS Linneeus. 
MYTILUS WHITEL Whitfield. 
Plate 37, fig. 9 a. 
Mytilus whitei Whitfield, 1877, Prelim. Rep. on the Pal. Black Hills, p. 18. 
Shell much higher than wide; basal margin regularly rotfnded ; pos- 
terior margin long, gently convex; front margin slightly convex below 
and concave above by the forward projection of the umbonal portion ot 
the shell; anterior portion of the dorsal margin straight, but the hinder 
portion sloping gently to the posterior margin; umbonal portion of the 
shell somewhat narrow and projecting beyond the front margin. Left 
valve moderately convex; surface marked by the usual lines of growth. 
Right valve and hinge unknown. 
Height from base to cardinal margin, 55 millimeters ; breadth of the 
body of the shell at about its mid-height, 32 millimeters. 
This shell is perhaps not a true Mytilus, but it is doubtless specifically . 
identical with the shell described by Professor Whitfield (loc. cit.) as M. 
whitei, the more elongate form being regarded as due to increased growth 
in that direction by age. 
Position and locality —Jurassic strata from near Fontanelle Caiion, 
upon the west side of Green River Basin, Western Wyoming, where it 
was collected by Dr. A. C. Peale. 
Genus TRIGONIA Bruguiere. 
TRIGONIA MONTANAENSIS Meek. 
Plate 38, fig. 2 a. 
Trigonia montanaensis Meek, 1873, An. Rep. U. S. Geol. Sur. Terr. for 1872, p. 472. 
Shell transversely elongate, somewhat arcuate and rudely subtetrahe- 
dral in marginal outline, gibbous anteriorly and narrowed and com- 
pressed behind; umbones elevated; from the umbo to the middle of the 
base the margin, as seen by side view, forms a continuous and almost 
true semicircular curve; from the middle of the base to the postero- 
basal angle the margin is slightly concave; posterior margin nearly 
