146 GEOLOGICAL SURVEY OF THE TERRITORIES. 
ruptly rounded up to the beaks; posterior margin narrowly rounded to 
the postero-dorsal margin; dorsal margin nearly straight or a little con- 
vex and moderately long; postero-dorsal margin gently convex and 
forming a steep slope to the posterior margin; valves strongly convex, 
especially about the middle; umbones compressed and very slightly 
elevated ; beaks proper, minute, and nearly terminal. The valves meet 
at a sharp angle along the postero dorsal slope, and both of them bend 
so abruptly inward below that the middle portion of the shell, when 
both valves are together, is nearly flat upon the under side. An ill- 
defined radiating furrow or depression upon each valve separates a small 
antero-basal portion from the remainder of the shell, behind which fur- 
rows or depressions its transverse thickness is greater than elsewhere. 
Surface marked by abundant, distinct, concentric lines and some coarser 
wrinkles of growth, but by no radiating lines. Its hinge has not been 
clearly seen, but it seems to be a true Volsella, and apparently does not 
belong to the subgenus Brachydontes of Swanison. 
Length, 58 millimeters; greatest width, which is about mid-length of 
the shell, 25 millimeters. This is the measurement of one of Mr. Meek’s 
types. The following is that of an example, less than adult size, from 
near Vermilion Cation, Northwestern Colorado: Length, 36 millime- 
ters; greatest width, 15 millimeters; greatest thickness, both valves 
together, 15 millimeters. This species, as pointed out by Mr. Meek, is 
very like the Modiola imbricata of Sowerby, ‘“‘ excepting that its anterior 
ventral portion, in front of the umbonal ridges, is more prominent, and 
its posterior basal extremity is more produced and narrowed.” 
Position and locality——Several specimens of this species were obtained 
by Dr. Peale from near the lower caiion of the Yellowstone River, and 
at Spring Cafon, in Montana; and a fineexample, not quite adult, was 
obtained by the writer of this article from near Vermilion Canon, North- 
western Colorado. 
VOLSELLA (MODIOLINA) PLATYNOTA (sp. nov.). 
Plate 37, figs. 3 a and b. 
Shell transversely subovate in marginal outline, prominently convex 
in the central and umbonal regions, in consequence of which the dorsal 
portion of each valve is so flattened horizontally as to give the shell a 
broad flattened back; basal margin broadly convex; posterior margin 
rounded, the postero-basal portion being most prominent, and the pos- 
tero-dorsal portion sloping upward and forward to the dorsal margin, 
which it meets with a moderate curve; dorsal margin nearly straight, 
its length being a little greater than half the full length of the shell; 
antero-basal portion of the shell a little compressed, its margin project- 
ing a little in advance of the beaks and somewhat abruptly rounded to 
the front; umbones narrow and moderately prominent; beaks project- 
ing very slightly beyond the immediately adjacent portion of the front 
border. 
Length, 35 millimeters; height at mid-length of the shell, 24 millime- 
_ ters; greatest thickness, both valvest ogether, which is about mid-length 
of the shell, 18 millimeters. 
This shell is evidently not a true Volsella, butitis believed to be conge- 
neric with the two species of Triassic shells which are described by Meek 
in vol. iv, p. 103, U.S. Geol. Sur. 40th Parallel (King), and doubtfully re- 
ferred by him to the Paleozoic genus Modiomorpha Hall & Whitfield. 
It is doubtless possible that a genus of conchifers should range from 
