168 
PALAEONTOLOGY OF NEW YORK. 
sub-triangular in outline, with their apices posterior, very convex on their 
anterior surface and elevated along their longest axis. The masticating 
surfaces are parallel, approximate, moderately broad and bear four or five 
denticles of which the posterior is apical. Their substance is tenuous as in 
the carapace, except upon the crown, where it is sufficiently thick to retain 
the form without distortion. A very large carapace, 90 mm. in length, 
which belonged to the largest individual yet observed, shows a single 
mandible somewhat out of place, and compressed in such a manner as to 
conceal the masticating edge. It shows, however, a long process at the 
anterior extremity (“ manubrium,” Beecher), which has been broken off at 
the tip. Such a process is often noticed in detached mandibles, to which 
reference will presently be made, but of the two specimens referred to in 
which these organs are undisturbed in the rock, the mandibles of the 
smaller evidently lacked any such processes, and those of the latter are 
so preserved as not to show them had they existed. These mandibles are 
situated within the posterior half of the carapace, and are at least one-third 
as long as the carapace itself. 
Abdomen long, slowly tapering to the caudal plate, and composed of six somites, 
which increase in length posteriorly, the ultimate joint being as long as the 
first two, and one-half longer than the penultimate. The anterior and pos¬ 
terior margins of each somite are slightly elevated and incurved on both 
dorsal and ventral sides; the dorsal surface is rounded and gently concave 
above, except upon the last somite. The posterior margin of each somite 
bears six short incurved spines, in three pairs. The strongest pair of these 
is upon the dorsal surface, a second and somewhat shorter pair upon the 
dorso-lateral surface, and a third pair (in young individuals often very 
faint) upon the lateral margins. Upon-the anterior somites these spines are 
usually quite small or visible only as nodes. The ventral surface bears 
no spines. The articulating surfaces of the somites are broad, and when 
the abdomen is normally extended they come into actual contact only at 
the sides, leaving both the dorsal and ventral margins distant, a fact which 
indicates (as noted by Beecher) great freedom of motion in these parts. 
