12 GEOLOGICAL SURVEY OF THE TERRITORIES. 
In form and general aspect this shell approaches that of a typical 
Ostrea, but in the character of its adductor scars, the extent of its plica- 
tions, and the subalation of its cardinal border, it is properly referable 
to the section Alectryonia. The only shell with which it need be com- 
pared is that of O. bellaplicata Shumard, also from Texas. It differs 
from that shell in being constantly larger, proportionally less capacious, 
broader towards the base, and in having its hinge-border longer and 
more oblique. 
Locality and position.—Cretaceous strata, Collin County, Texas, where 
it was collected by Mr. 8S. W. Black, and sent by him to the Smithsonian 
Institution. The specific name is given in his honor. 
Genus EXOGYRA Say. 
EXOGYRA WINCHELLI White. 
Plate 13, figs. 1a, b, ¢, and d. 
Exogyra winchelli White, May, 1880, Proc. U. S. National Museum, vol. ii, p. 294. 
Shell of medium size, irregularly subovate in marginal outline; sessile, 
or attached by a.large part of the surface of the lower or left valve, be- 
ing obliquely inclined so that the anterior border is very much higher ° 
than the posterior. Lower valve massive, moderately deep, its front side 
nearly perpendicular and of considerable height vertically; umbo ver- 
tically flattened continuously with the front side, and broadly curving 
backward; beak closely incurved under the posterior border and con- 
cealed; ligamental groove long and narrow, occupying the whole curva- 
ture of the umbo. Upper valve nearly flat, thick, the anterior part be- — 
ing much thicker than the posterior; beak vertically thin or compressed, 
closely coiled in a plane with that of the valve, making a little more 
than one entire volution. Surface marked by coarse lines of growth, 
and, near the anterior borders of both valves, especially the upper, it is 
usually deeply laciniate. . 
Length, 90 millimeters; breadth, 66 millimeters; height in front, 55 
millimeters. 
This species belongs to the same section of the genus Hxogyra with 
EH. haliotoidea Sowerly sp. and H. walkert White. The latter species is 
larger and proportionally broader than H. winchelli and not properly 
sessile, as the latter species is. H. haliotoidea, as figured by d’ Orbigny 
in Pal. Franeaise, t. iii, pl. 478, differs from 2. winchelli in being propor- 
tionally higher in front and narrower in transverse diameter, and in not 
having the beak so much ineurved. LH. interrupta Conrad, from Missis- 
Sippi, also belongs to the same section, but that species is described as 
having radiating ribs, which H. winchelli has not. 
Position and locality.— Cretaceous strata, Collin County, Texas, where 
it was collected and sent to the Smithsonian Institution by Mr. 8. W. 
Black. The collections of the Institution also contain a fine example 
sent by Prof. A. Winchell many years ago from Prairie Bluffs, Ala., 
which is believed to be specifically identical with the form here described, 
but it is proportionally more elongate, has a larger muscular scar, and 
the umbonal curve is a little more abrupt. The specific name is given 
in honor of Professor Winchell. 
