150 PALEOZOIC PALEONTOLOGY. 
arched over near the beak by a small deltidium, and nearly closed 
between this and the hinge margin by the prominent cardinal process 
and chilidium of the other valve. Brachial valve concave, its greatest 
concavity near the middle, following nearly the curve of the pedicle 
yalve, so as to leave but a narrow, visceral cavity within; beak not 
distinct from the cardinal margin; cardinal area narrow or nearly 
linear, lying at a right angle to the plane of the valve. Surface of 
both valves marked by numerous, fine, closely-arranged, equal, radi- 
ating coste, or with every fourth, fifth or sixth one larger and more 
prominent than those between. 
The dimensions of a very large specimen are: length, 15 mm. 3; 
width, 23 mm. 
Remarks.—This a variable species, especially in regard to its size. 
In the Trenton limestone of New Jersey two forms are conspicuous, 
which are usually not associated in the same bed. A large form, 
having a width of 20 mm. or more, occurs usually in the lower strata, 
while higher up a smaller form, from 12 to 15 mm. in width, is 
abundant, often constituting almost entirely some of the thinner | 
strata. The large form in the lower beds is frequently marked by 
several folds or wrinkles, of greater or less strength, on each side of 
the beak, which extend from the cardinal margin obliquely toward 
the median line of the shell. This species is probably more abundantly 
represented in the Trenton fauna of New Jersey than any other. 
STROPHOMENA INCURVATA (Shep.). 
Plate IX., Figs. 16-17. 
1892. Strophomena filitexta Hall and Clarke, Pal. N. Y., vol. VIIT., 
Dilly sls Or tessa 7, mpl. Oise igest * is Ses 
Description—Shell wider than long, the greatest width along the 
hinge-line, more or less strongly concavo-convex, subsemi-elliptical in 
outline; cardinal extremities angular and deflected, lateral and an- 
terior margins regularly rounded. Pedicle valve slightly convex in 
the umbonal region, but otherwise more or less deeply concave, often 
with an ill-defined sinus toward the front; the beak extending but 
little beyond the cardinal margin, minutely perforated; cardinal area 
variable in width and elevation, slightly retrorse in very gibbous speci- 
