34 JRetnai'ks on the Trilohite. 



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 curious, 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 guUar 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. 



The 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 naairalist, 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 guUar pouch 



