616 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
of Neostrophia gregaria from the Bromide formation at Rock Crossing, Okla., 
but it can be readily differentiated from any of the known camerellids except 
Metacamarella, Salonia, and Parallelelasma by its brachial interior. 
Didymelasma differs from Metacamarella in the form of the valves, the latter 
genus being elongate and flaring anteriorly. It differs internally in having a long 
and nearly straight brachial process rather than a long, curved one as described 
for Metacamarella. 
Salonia is totally unlike Didymelasma in its transverse form and the strong 
anterior lobation. The brachial interior resembles that of Salonia in the presence 
of the long brachial process, but that structure in Didymelasma is given off from 
a more anterior point than in Salonia and the process is nearly straight. 
The exterior of Parallelelasma has a distinctly pentagonal outline with the 
greatest width anterior to the middle and the shell strongly angulated at the place 
of maximum width. In these respects it is entirely unlike Didymelasma. Inside 
the brachial valve the position and great length of the brachial process in Didy- 
melasma are important differences. 
DIDYMELASMA ABRUPTUM Cooper, new species 
Plate 120, E, figures 30-33 
Shell small, subcircular in outline with length and width nearly equal; sides 
rounded ; anterior commissure uniplicate ; surface smooth. 
Pedicle valve moderately convex in lateral profile and fairly strongly convex 
in anterior profile. Umbonal, median, and flank regions swollen ; sulcus originat- 
ing about 14 mm. anterior to the beak, shallow and narrow but widening an- 
teriorly although not deepening greatly in the same direction. Old specimens 
with a faint costa at the anterior end of the sulcus. Tongue short. Interior with 
a median septum extending to about the middle. 
Brachial valve gently convex in lateral profile, greatest convexity in the pos- 
terior third, anterior third somewhat flattened. Anterior profile broadly convex ; 
umbonal and median regions full. Sulcus originating between the umbo and the 
middle, fairly strongly elevated at the front, flattened on top. Flanks swollen 
but strongly depressed below the level of the fold. Interior with two short, sub- 
parallel plates. 
Measurements in mm.— 
Brachial 
Length length Width Thickness 
Pedicle valve (C.U. 26403(928)a)......... 5.5 ? 5.8 1.9? 
Brachial valve (C.U. 26403(928)b)........ ? 5.3 5.3 1.7 
( ©) GG. 26403 (028) ©) iit insta ss. 4 5.8 6.1 ? 
Types.—Holotype: Columbia Univ. 26403 (928)b; figured paratypes: Colum- 
bia Univ. 26403 (928)a,c. 
Horizon and locality—Rockland formation in Ontario, Canada: Northeast of 
Gretna. 
Discussion.—This is a larger species than the D. longicrurum of the Lebanon 
formation. Besides size it differs in the fold and sulcus, those of the Ontario form 
