116 
PALAEONTOLOGY OF NEW YORK. 
nearly devoid of markings of any kind. There is no apical foraminal tube as 
in Retzia, no dental lamellae, or muscular ridges, and only in extremely rare 
instances is there any trace of the muscular impressions. 
In the brachial valve the structure of the hinge-plate is very complicated. 
It may be described as composed of two parts, a posterior and an anterior. 
The posterior portion is rather broadly crescentic in form, having the curvature 
of the umbonal margin of the valve; its lateral extensions form the socket- 
walls, which are moderately broad, deep and well defined. On the central por¬ 
tion of this part of the plate rests a second crescent, having its horns, which 
make nearly a semicircle, directed backward and into the umbonal cavity of the 
opposite valve. The curvature of the plate at this point is such that the 
base of the crescent lies upon the inner surface of the deltidium of the pedicle- 
valve, its horns crossing the deltidium, extending for fully half the length of the 
umbonal cavity, and being elevated at their tips, above the inner surface of the 
shell. The posterior part of the hinge-plate is connected with the anterior 
part only by a narrow, thickened median band, which is constricted transversely 
at the point of union, the transverse groove being crossed only by a very fine 
axial ridge. The anterior portion of this plate consists of a long, narrow, tri¬ 
angular, concave or spoon-shaped central process, the edges of which are sharply 
elevated, and the extremities of these marginal ridges are produced into two long, 
slender and nearly vertical crura. The hinge-plate is not connected with the 
lateral portions of the shell, but is supported by two slightly divergent, upright 
laminse which extend to the bottom of the valve; and since the valve is deep in 
this region, these laminm are very conspicuous. The crura widen as they ap¬ 
proach the apices of the primary lamellfe, and form their union with these by a 
short, abrupt lateral curve. The spiral cones are approximate, their apices lateral, 
the ribbon making eight or nine volutions in the adult individual. The umbonal 
blades are broad for one-third their length, narrowing abruptly in front of the 
loop. The loop is situated posteriorly, and is constructed as follows ; two slen¬ 
der lateral processes are given off from the primary lamellae, which are directed 
forward in low, downward curves, and near their extremities turn inward 
and upward, meeting at a point just behind the center of the shell-cavity. 
