150 GEOLOGICAL SURVEY OF THE TERRITORIES. 
strata. It is distinguished from other known species by its inconspicu- 
ous beaks and its obliquely downward and forward truncation of the 
upper portion of the front. 
Position and locality.—Jurassic strata near the lower caiion of Yellow- 
stone River, Montana; where it-was collected by Dr. A. C. Peale. 
Genus TANCREDIA Lycett. 
TANCREDIA EXTENSA (Sp. nOv.). 
Plate 38, fig. 4 a. 
Shell transversely elongate, the length being about double the height; 
moderately and somewhat regularly convex, but the umbones are some- 
what prominent, and there is a slight lateral flattening of the postero- 
dorsal portion of each valve; basal margin forming a broadly-convex 
curve; both anterior and posterior margins narrowly rounded ; postero- 
dorsal margin sloping downward and backward from the beaks with a‘ 
very gentle convexity; antero-dorsal margin coneave in front of the 
beaks and gently convex further forward; beaks situated at about mid- 
length of the shell. Surface marked by concentric lines of growth. 
Length, 58 millimeters; height from base to beaks, 29 millimeters. 
The only examples of this species that have been discovered are 
in the form of natural casts in sandstone. It is doubtless congeneric 
with those American Jurassic species which have been very generally 
referred to Tancredia, but this reference of it is made wholly from exter- 
nal characters, the hinge and interior markings being unknown. It 
somewhat resembles T. inornata (= Astarte inornata Meek & Hayden), 
but it is a larger and much more elongate form. 
Position and locatity.— Jurassic rocks, north side of Bull Lake Fork, 
Southeastern Idaho, where it was discovered by Prof. O. St. John, in the 
season of 1878. 
Genus PHOLADOMYA Sowerby. 
PHOLADOMYA KINGII Meek. 
Plate 38, figs. 3 a and DB. 
Pholadomya kingit Meek, 1873, An. Rep. U. S. Geol. Sur. Terr. for 1872, p. 473. 
Shell elongate-oblong in marginal outline, excluding the elevated um- 
bones, moderateiy inflated, laterally constricted behind and slightly gap- 
ings basal margin broadly convex or slightly flattened a little in front 
of its mid-length; front and posterior margins both regularly rounded ; 
dorsal margin nearly straight, but the dorsum appears to be strongly 
concave because of the conspicuous elevation of the umbones; beaks in- 
curved, approximate, and situated very near the front. Surface of each 
valve, except a short space at the front and the whole postero-dorsal 
space, marked by twelve or fuurteen slender radiating costz of the 
usual character, the first one being vertical, and the others successively 
increasing in obliquity of direction to the last, which ends at the postero- 
basal margin. 
Length, 46 millimeters; height from base to dorsal margin, not in- 
