328 PALEOZOIC PALEONTOLOGY. 
only to about the middle of the valve; beak acute, erect. Brachial 
valve about equally convex with the pedicle, flattened in the centre, 
the lateral slopes curving rather abruptly to the postero-lateral mar- 
gins; slightly depressed along the median line near the beak, but 
elevated toward the front in an ill-defined mesial fold scarcely higher 
than the general surface. Surface of each valve marked by twelve or 
thirteen angular, radiating plications. On the pedicle valve the median 
plication bifurcates near the beak, and near the centre of the valve an 
additional plication is intercalated between these two divisions. On the 
brachial valve the median plication bifurcates near the centre of the 
valve. The remaining plications on both valves are simple and con- 
tinue to the beak. 
The dimensions of an average specimen are: length, 6 mm.; width, 
5 mm.; thickness, 3 mm. 
Remarks.—The specimens which have been identified as R. bialveata 
agree closely with Hall’s original description and illustrations of the 
species in general form, size and proportions, but the peculiar bifur- 
cations of the central plications of each valve of the New Jersey shell 
is not as described by Hall. The number of New Jersey specimens 
available for study is small, and it is quite probable that with a larger 
number of specimens this character would be found to be more or less 
variable. 
RHYNCHOTREMA FORMOSA (Hall). 
Plate XLII., Figs. 5-8. 
See, also, p. 309, pl. XXXVI. 
1859. Rhynchonella formosa Hall, Pal. N. Y., vol. IIL., p. 236, pl. 
35, figs. 6 a—y. 
Description.—Shell broadly suboval, subtrigonal or subrhomboidal 
in outline, usually broader than long; the cardinal margins meeting 
at the beak in an angle varying from 78° to 110°. Pedicle valve 
convex; the surface curving rather abruptly from the umbo to the 
cardinal margins and more gently laterally; the median portion de- 
pressed in a more or less abrupt sinus, which does not reach quite to 
the beak; the beak prominent, moderately incurved. Brachial valve 
more strongly convex than the pedicle, its surface curving gently to 
the lateral margins; the mesial fold more or less abruptly elevated 
