This fine genus of trilobites was established November, 1824, by my friend James E. Dekay, M. D. It embraces a considerable number of species so analogous to each other, that except in a very few cases, it is exceedingly difficult to point out their distinctive characters. Some of the species of Isotelus, appear to have reached a greater size than any other trilobite. In the cabinet of P. A. Browne, Esq., there is the fragment of one, which must have been fourteen or fifteen inches long. The Isotelus is found in several parts of North America, but most abundantly in the black transition limestone, in the northern section of the country. The richest locality, not only of this genus, but also of the Calymene and the Asaph, is Trenton Falls, on West Canada Creek, about 13 miles to the north of Utica, in the state of New York. The following extracts from the notes of Professor Renwick, which accompany Dr. Dekay's account of the Isotelus, will give some idea of this vast depository of the medals of ancient zoology. West Canada Creek, is one of the principal branches of the Mohawk River. At Trenton Falls it has worn itself a passage through the rock for the distance of nearly two miles, forming a series of water falls; and has thus laid open to view the strata to the depth of probably 300 feet. The layers of the rock thus disclosed are nearly horizontal, and of various thicknesses: they are composed of limestone, with the exception of numerous thin veins of argillaceous matter. The higher strata are composed of carbonate of lime nearly pure, of a light grey colour and crystalline structure. At greater depths it is more compact and darker in colour, and finally it appears quite black and highly fœtid.[36] [Footnote 36: See Annals of N. Y. Lyceum, vol. i. page 185.] Animal remains are contained in every part of the rock; besides several genera of trilobites, we have several species of orthocera. Encrinites and Fungites--Nautili--Terebratulæ and Producti, are quite common. The favosites here are sometimes six inches in diameter, and in such numerous columns, as to have induced the late worthy proprietor of this interesting spot, Mr. J. Sherman, to consider them as analogous in structure to the basaltic columns of Staffa and the Giant's Causeway; he therefore maintains the extravagant theory that these columns are nothing more than gigantic favosites.[37] We visited this famous locality of trilobites not long since, and were almost as much delighted with the sublimity and grandeur of the cataract, and the picturesque and romantic character of the glen, as with the reliques of olden times, which are scattered here in such profusion. [Footnote 37: See a Description of Trenton Falls, by John Sherman, p. 17.] The genus Isotelus, derived from Ἱσος, equal, and τελος, extremity, is thus characterized by Dr. Dekay. _Body_ oval often contracted, not unfrequently extended.