WUITE.] CRETACEOUS FOSSILS. 11 
any visible scar of attachment, varying from ovate to circular; rounded 
or sometimes a little straightened on. the hinge-margin; beaks hardly 
projecting beyond the outline of the cardinal margin. Lower valve very 
shallow; cartilage-pit unusually small, shallow, and short. Upper valve 
almost perfectly flat; cartilage attachment even shorter than that of the 
other valve, and slightly convex on its inner margin. Muscular scars 
unknown; surface of both valves with small, regular concentric wrinkles 
most distinctly marked on the central region. 
“‘ Greatest diameter of one of the largest oval specimens, 1.70 inches; 
breadth, 1.40 inches; convexity, 0.23 inch. 
‘This species is remarkable for the thinness of the shell, the slight 
convexity of the under valve, and the flatness of the upper, as well as 
for its rounded or slightly straightened cardinal margin, and the absence 
of any scar of attachment, or of any traces of muscular impressions 
within. These external characters, and.the regular small concentric 
wrinkles, give the exterior of lower valves of circular specimens some- 
what the appearance of a Lucina or Dosinia, while in other individuals 
it looks more like an Anomia or Placuna. 
“Locality and position.—Missouri River, below Gallatin City, Mont. 
Cretaceous.” 
The comparative smoothness of these shells is so unusual with mature 
shells of any of the Ostreide as to suggest the possibility that they are 
immature examples; but their considerable abundance, and the nearly 
uniform size of all the specimens, seems to indicate that they are adult 
shells. If so, they evidently do not belong to the typical section of the 
genus Ostrea; but without some knowledge of the character of the mus- 
cular markings, it is unprofitable to offer any suggestions as to their 
true generic affinities. 
Subgenus ALECTRYONTA Fischer: 
OSTREA (ALECTRYONIA) BLACKII White. 
Plate 14, figs. 1a and b; and Plate 17, fig. 4a. 
Ostrea blackii White, May, 1880, Proc. U. S. National Museum, vol. ii, p. 293. 
Shell irregularly subovate in marginal outline, moderately capacious; 
beaks small, sometimes obscure and sometimes moderately prominent, 
pointing a little backward, but not conspicuously so. Lower valve 
usually moderately deep and capacious, its convexity being more prom- 
inent about the middle than elsewhere, often subalate, but the latter 
feature is usually obscure; scar of attachment at the beak usually 
present, and often moderately large; ligament-area usually short and 
rather small, but sometimes comparatively large and laterally extended; 
ligament-furrow well defined, and of the usual character. Upper valve 
nearly flat, and corresponding with the lower in other respects, except 
that it is not so broad at the hinge border, and never has there the 
subalations which sometimes mark the lower valve. The adductor scars 
are moderately large, and of the form common to Alectryenia, namely, 
curved-spatulate. Surface of both valves marked by concentric lines 
and strong imbrications of growth, and each by a dozen or more radi- 
ating ribs or plications, which constitute a conspicuous feature of the 
Shell, but they are usually somewhat less distinct upon the upper than 
upon the lower valve. 
Length, 68 millimeters; greatest breadth, 62 miliimeters; thickness, 
32 millimeters. 
