52 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 134 
the fold and sulcus to be essentially the same as that of the American 
specimens, with the exception that the intercalated costae on each 
side of the median 2 in the sulcus are longer than in the Russian 
specimen of comparable size. Furthermore, the costae of the Oregon 
specimens are somewhat more slender than those of the Russian 
specimens. In Russia the species attains a width of about 1 inch or 
about twice the size of the Oregon specimens. 
STENOSCISMA AMERICANUM Cooper, new species 
Plate 9D, figures 29-33 
Shell small, valves of subequal depth, subtriangular in outline, 
incomplete (lacking the frills), slightly wider than long. Postero- 
lateral margins straight, lateral margins narrowly rounded, anterior 
margin truncated; valves paucicostate, the umbo of both valves 
smooth but the remainder costate. Costae numbering about 15 or 16, 
4 on the fold, 1 on each side of the fold, and 4 or 5 on the flanks. 
Pedicle valve slightly less deep than the brachial valve, gently 
convex in lateral profile, flattened in anterior profile; sulcus orig- 
inating just anterior to the umbo, shallow but wide, occupying more 
than half the width at the front; sulcus occupied by 3 primary cos- 
tae, but 1 or 2 costae are intercalated on the slope on each side of 
the primary 3; tongue bent nearly at a right angle, long, and broadly 
rounded ; flanks bounding sulcus narrowly rounded. 
Brachial valve gently convex in lateral profile, strongly rounded 
in anterior profile; fold originating just anterior to the smooth umbo, 
low but wide, and occupied by 4 primary costae. Secondary costae 
intercalated on sloping sides of fold; flanks bounding fold convex 
but depressed below the fold; umbo swollen. 
Measurements in mm.—Holotype, length 11.2, brachial length 9.7, 
width 12.0, thickness 9.0. 
Type.—Holotype, U.S.N.M. 125381. 
Locality.— S20. 
Discussion—This species is similar to S. mutabile oregonense but 
differs in its ornamentation and the smooth umbones of both valves. 
It is similar to S. karpinskyi (Tschernyschew) but differs in having 
smooth umbones rather than plicated ones and the details of the 
ornamentation are somewhat different. On the pedicle valve of S. 
americanum, new species, the fold has only 4 costae but 2 additional 
ones are intercalated on the sloping side of the fold, unlike S. karpin- 
skyi as illustrated by Tschernyschew (1902, pl. 60, figs. 2, 3), in 
