_Head_ or buckler large and rounded, equalling the tail in size, but with two oculiform tubercles. _Abdomen_ with eight articulations. Frontal process beneath, with two semilunar terminations. _Post abdomen_ or tail, broad, expanded with indistinct divisions, as large as the buckler. Longitudinal lobes very distinct. Other distinguishing marks by which this genus may be known, have been given in our introduction. Isotelus Gigas. _Dekay_. Casts Nos. 21 and 22. _Head_ representing a spherical triangle, surface punctate, convex, descending from between the eyes to the anterior border, which has a narrow raised rim; posterior extremity concave and corresponding to the articulation of the abdomen. Eyes elevated, prominent, sub-pedunculated; cornea oblong, lunated, highly polished; _abdomen_ with eight distinct articulations, the middle lobe double the size of the lateral one: these latter are continuous with the middle lobe, have a deep furrow impressed on their upper surfaces, which becomes gradually effaced towards their narrow free extremities. These lateral lobes are rounded at their extremities, and flattened in such a manner as to allow each lobe to slide easily under the lobe immediately preceding. _Tail_ subtriangular, convex, equalling the head in size, with the posterior termination rounded. On the centre of its surface, when accidentally decorticated, a slight elevation may be traced, if the specimen be held in a certain light, which appears to be a continuation of the middle lobe; this extends to within a short distance of the posterior angle of the tail, when it is either entirely effaced or terminates in an abrupt truncation. Another elevation runs parallel to and at a short distance from the edge of the tail. These elevations are connected by obscure parallel lines, imitating the spaces between the lateral lobes. When the tail is fractured on the borders, a semilunar depression is visible, exhibiting concentric striæ. The whole Surface of the animal has a jet black polish. Length from 6 to 12 inches. The original of our cast is in the cabinet of J. P. Wetherill, and was found near Cincinnati, Ohio. It is of a yellowish colour, and occurs in argillaceous slate. Specimens are common in most cabinets of American fossils. The Lyceum in New York, possesses a fragment of an individual of this species, which must have been at least 17 inches long. Our cast, No. 22, is from the gigantic tail in the cabinet of P. A. Brown, Esq. Mr. Stokes describes the I. gigas as a new species under the name of Asaphus Platycephalus, in Geolog. Trans. vol. i. N. Series. His specimen was found in the limestone of. St. Joseph's, Canada. Isotelus Planus.[38] _Dekay._ Cast No. 23.