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Remarks on the Trilobite. 37 
being reflected beneath the animal so as to form a flat horizontal 
plane, which terminates in a kind of lunate spine, the horns of 
the crescent being curved towards each other. These horus are 
six lines in length, and their points are sharp and translucent. 
We have received from Dr. Warder a specimen of this singular 
structure, which was found, with other fragments of the isotelus, 
near Springfield in Ohio. Although it lies on the rock, unaccom- 
panied by any other fragment of the animal, its exact resemblance 
to the figure given by Dr. Dekay leaves no doubt that it once 
belonged to an isotelus. Among other conjectures respecting the 
uses of this crescent-shaped structure, it is observed that when 
the animal was attacked “it may roll itself up into a ball, as 
indeed it is often found, and by some mechanism these processes 
may be inserted into the corresponding cavities in the tail, and 
thus retain permanently a rolled position, presenting nothing but 
its calcareous covering to the enemy; or they may supply the 
place of antenne, for which their form and contiguity to the 
mouth and brain would seem to render them peculiarly applica- 
ble.” The first conjecture above noticed was ingenious, and will 
no doubt be confirmed when the lower surface of the tail is dis- 
covered. The inferior organization of the calymene bufo has at 
any rate given great plausibility to this opinion. 
We have also carefully examined another fragment represent- 
ing a similar structure. The original fossil was found in Ohio, 
and is now in the possession of W. Wagner, Esq. of Philadel- 
phia. The rock on which it oceurs is a gray limestone full of 
other petrifactions. This lunate structure differs essentially from 
the one noticed by Dr. Dekay; the points of the crescent are 
rounded and do not curve towards each other ; the terminations 
are not raised and translucent, but the whole surface is nearly 
flat.. It however formed, undoubtedly, a portion of the under 
Surface of some trilobite, whether that of an asaphus, an isotelus, 
ora dipleura, we are unable now to determine. In the Geol. 
Trans., No. 8, Vol. I, pl. 27, there isa figure by Mr. Stokes of 
What is said to be the under surface of the anterior portion of the 
Shield of an asaphus platycephalus from Lake Huron. Dr. Buck- 
land, whose copy of the figure we have only seen, observes con- 
Cerhing it, that the entrance to the stomach of the animal was 
between these lunate processes “analogous to that in recent 
crabs.” The A. phatycephalus is synonymous with I. gigas of 
