42 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 134 
MARGINIFERA? COSTELLATA Cooper, new species 
Plate 6C, figures 22-26 
Shell small to medium size for the genus, transversely subrec- 
tangular in outline. Hinge forming widest part; lateral margins 
directed toward the valve middle, slightly convex; anterior margin 
broadly rounded. Surface marked by subdued, rounded costellae 
separated by spaces of much less width than the costellae. Visceral 
disk not strongly reticulated. Spines few, but 4 large ones located 
in a horizontal row on flanks and trail. 
Pedicle valve having a visceral area about 10 mm. long, after 
which the valve is strongly geniculated. Trail moderately convex 
in profile and about 14 times as long as the visceral area. Beak 
protruding slightly posterior to the hinge; umbo somewhat swollen, 
narrow; visceral region inflated; umbonal slopes and slopes of 
flanks of visceral region moderately steep. Ears small but prom- 
inent, narrowly rounded, forming right angles; sulcus originating 
about 3 mm. anterior to the beak, extending to the anterior margin 
as a trough, deep and narrowly U-shaped in profile. Flanks nar- 
rowly rounded and with steep slopes to the margins. Four long 
spines, two on each side of the sulcus, given off at nearly a right 
angle from flanks bounding sulcus of trail and forming a hori- 
zontal row. 
Brachial valve closely fitting the pedicle valve, deeply concave 
at the point of geniculation; umbo concave; fold originating about 
3 to 4 mm. anterior to the umbo, moderately narrowly rounded; 
flanks moderately concave; ears deflected and flattened. 
Interior —A longitudinal section of a complete specimen gives 
no indication of a marginal rim. 
Measurements in mm.—Holotype, surface length 28, length 13.6, 
length of visceral area 9, hinge width 18.6; height Io. 
Types.—Holotype, U.S.N.M. 125368; figured paratype, U.S. 
N.M. 125370a; unfigured paratypes, U.S.N.M. 125370b-c. 
Localities —S39, S43, S46; U.O. 2202-2. 
Discussion—This species is characterized by its strong costellae 
and deep sulcus. It is similar to Probolionia posteroreticulata, new 
species, in exterior appearance but is wider, has a less reticulate 
visceral disk region, and the sulcus is broader. The interior is not 
definitely known and a margin was not seen, nor is it indicated on 
the exterior. 
~ cA 
es = ——E—E———————E—————— ee 
