The buckler is subtriangular, and covered with granulations; the anterior portion of our specimen being mutilated, we cannot determine its form exactly. The cheeks are very prominent, and swell up gradually towards the oculiferous protuberances, which are oblique, and marked at their apex with a depression, so as to give them an annular appearance. The abdomen is crossed by fourteen double distinct articulations, not interrupted in their course, by the two longitudinal furrows, so common in most of the trilobites; but owing to certain curves or irregularities in the ribs near their lateral termination, a trilobate appearance may in some specimens be detected. Tail suborbicular, convex, and covered with a thick epidermis. The specific name of this species was given in compliment to Dr. James E. Dekay, of New York, whose valuable paper on the genus Isotelus, first directed my attention to the American trilobites. The D. Dekayi has been found in several districts of the United States; at Lockport in the State of New York, it is not uncommon. The small specimen from which our cast of the abdomen and caudal end was taken, is in the fine cabinet of Mr. William Hyde, who permitted me to use it with his wonted liberality and kindness. It is said to have been found in Northumberland, Pennsylvania, and occurs in grey carbonate of lime. In the Philadelphia Museum, there is a fine fragment of this species, in which there is embedded some crystals of iron pyrites; it was obtained in Ulster County, New York. In the cabinet of the Academy of Natural Sciences, there is a longitudinal and hollow fragment, filled with ochre, and the oxide of iron; it is labelled from Lockport, New York. At Mount Hope Institution, near Baltimore, there is also a good specimen from the same locality. In the _Clinton collection_, owned by the Albany Institute, there is a large extended fragment, nearly five inches long. It is embedded in brown limestone, and was found in Madison County, N. Y. There are twelve articulations of the abdomen remaining, and the epidermal covering of the tail is distinctly marked with numerous dots. In the same collection there is another large fragment of this species, consisting of the tail and fourteen articulations. It was found in Steuben County, New York; and occurs in grey limestone. It is slightly contracted and very much depressed laterally. There is also a head in the same kind of limestone, from Cazenovia, Madison County, New York. In the cabinet of the Institute there is another specimen of this species, about six inches in length, and nearly perfect; it is also embedded in a similar rock, and was brought from Rochester, Munroe County, New York. The original of the head from which our cast was made, is in the cabinet of P. A. Browne, Esq., and was found by that enterprising geologist near Lehighton, Pa. Genus Trimerus.[46] _Green._ [Footnote 46: From the Greek for "three divisions."] _Body_, contractile, tapering, compressed.