34 Remarks on the Trilobite. 
tory of fossil crustacea, seems to have ascertained by his useful 
and ingenious researches, that the irregularities of the external 
shells in the living species of crustaceans have a constant relation 
to distinct compartments in their internal organization, and by 
the application of these distinctions to fossil species, he has been 
enabled to draw some highly curions, novel, and important con- 
clusions respecting their internal and general structure. From 
my limited knowledge of the anatomy and the habits of our 
living crabs, I would merely suggest, that the peculiar organ in 
the animal economy of the trilobite, which the gullar plate above 
described, was intended to model and protect, was perhaps the 
stomach, and that the spaces on each side covered the anterior 
portions of the liver. 
he upper shell of the genus calymene, like that of the iso- 
telus and depleuva, naturally and obviously divides itself into 
three parts, the buckler or shield—the abdomen and the caudal 
end. This last portion in the calymene is not covered with a 
thick epidermis, as in the two genera above mentioned, the ar- 
ticulations being all visible and somewhat difficult, in some spe- 
cies, to distinguish from those of the abdomen. These articula- 
tions, which are generally ten in number, are composed of a 
variety of immovable plates as in the other genera. The infe- 
rior surface of the caudal. end of the trilobite had never been 
observed by any naturalist, till my friend Dr. Cohen, obtained 
some fragments of the genus calymene from the neighborhood of 
Berkley Springs, in Virginia, in some of which that structure 
was developed. These were kindly sent to me for examination, 
along with those of the buckler just described. 
From our researches we have ascertained, that the inflexible 
margin which surrounds the caudal end or tail of the calymene 
bufo, is not reflected beneath the body of the animal, as might 
be expected, but that there is joined to it by a structure a slightly 
concave horizontal surface. This surface is lunate, being broader 
below the articulations of the vertebral column, gradually dimin- 
ishing on each side towards the horns of the crescent, which 
terminates just below the last articulations of the abdomen. This 
lunate surface is composed of a thick crustaceous plate or piece. 
Beyond this crescent shaped piece, directly below the vertebral 
column, there is a deep cavity in the under shell of the animal, 
which corresponds in figure and dimensions with the gullar pouch 
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