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158 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 
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proportionally larger wings, narrower body, and a more nearly erect 
axis. 
Position and locality—Lower Potato Valley, Southern Utah, where it 
was obtained by Prof. J. W. Powell from Cretaceous strata. 
Genus SOLEMYA Lamarck. 
SOLEMYA BILIX (Sp. nov.). 
Shell about two and a half times as long as high, broader anteriorly 
than posteriorly; both ends rounded, the posterior one more narrowly 
so than the other; both dorsal and basal margins gently convex or nearly 
straight; test thin and fragile; valves moderately convex from above 
downward, the greatest convexity in that direction being near the dor- 
sum; beaks, having the usual inconspicuous character common to the 
genus, situated near the posterior end; ligament necessarily short, but 
apparently well developed, and resting upon a fulerum of support of the 
usual character in each valve. Surface bright and, besides the usual 
lines of growth, marked by numerous faint radiating lines, which are 
visible to the unassisted eye, but are satisfactorily seen only under a 
lens, nearly uniformly distributed over the whole surface, but upon the 
middle portion they are arranged in pairs. 
Length, 20 millimeters; height at the broadest part, which is in front 
of the middie, 8 millimetres. (Museum No. 8913.) 
This is plainly a characteristic species of Solemya, but the only fossil 
species with which it need be compared is S. subplicata Meek & Hay- 
den, from the Fox Hills Cretaceous of the Upper Missouri. It differs 
from. that species in being proportionally broader in front, in the charac- 
ter and uniformity of distribution of its radiating striae, and in wanting 
. the subplicate character of the front portion. The extension of the epi- 
dermis has not been observed, but in other respects this species may be 
compared with the living S. velum Say in general form, and in the pair- 
ing of its radiating lines. 
Position and locality.—Cretaceous strata, associated with Mactra holmesit 
(= Cyrena? holmesii Meek), about four miles north of Golden, Colo., 
where it was obtained by Mr. W. H. Holmes. These strata were for- 
merly supposed to belong to the Lignite series (Laramie), but they are 
marine Cretaceous, as I have shown in An. Rep. U. 8. Geol. Sur. Terr. 
for 1877, pp. 193-196. 
Genus LUCINA Bruguitre. 
LUCINA PROFUNDA (sp. nov.). 
Shell subcireular or subpentahedral; valves not very convex; pos- 
terior side truncate, narrower than the anterior; basal border having its 
margin more abruptly convex at its middle than towards the front and 
rear; dorsal margin short, nearly straight; front margin having a nearly 
