_Buckler_, pustulous, indistinctly lobate, with only two small elevated oculiferous tubercles. _Abdomen_, with thirteen distinct, double articulations divided into three lobes by a slight longitudinal furrow. _Flanks_, or lateral lobes, not so broad as the middle lobe. _Tail_, tapering to an obtuse point, pustulous, and marked with ten articulations. This genus resembles in some respects both the Calymene and Dipleura. The form of the buckler, the position and structure of the oculiferous tubercles, and the scarcely lobate divisions of the abdomen, will readily distinguish it from the Calymenes. The articulations of the tail, not being covered with a shelly crust, is a character too obvious to confound it with the genus Dipleura. There is, we think, a beautiful chain of gradations of resemblances, between the Isotelus, Dipleura, Hemicrypturus and Trimerus. The lobes of the abdomen of the Isotelus are very distinct, and the articulations of the tail are hid by a broad thick shelly crust. The lobes in the Dipleura are scarcely apparent, the ribs more numerous; and the covering of the tail much smaller. The lobes of the Hemicrypturus are like those of the Isotelus; but the lateral ones only of the tail are covered. In the genus Trimerus the lobes are like those of Dipleura, but the articulations of the tail are exposed. Trimerus Delphinocephalus. _Green._ Cast No. 32, and Fig. 1. Clypeo semilunari, antice compresso; oculis minimis, enimentissimis; articulis duplicibus vix lobatis; cauda attenuata; corpore tuberculata. In the rich cabinet of American fossils in the Albany Institute, there are two fine specimens of this species, and I am indebted to that rising and liberal institution, for the use of them in the present work. Our cast is made from the smaller and more perfect specimen of the two. The outline of the buckler forms an irregular semi-ellipse. The front is convex between the eyes, and very much depressed anteriorly, so as to form a sharp edge. The posterior part of the buckler is marked with a transverse groove parallel with the articulations of the back. The cheeks are small and triangular; the small elevated eye-shaped tubercles being placed in the middle, nearly equidistant from each of the angles. The eyes are not reticulated, the summit of each tubercle only presenting a plain oval foramen. The middle lobe of the abdomen is much broader than the lateral lobes, and has 13 distinct, double articulations. The side lobes are curved, and each costal arch is flattened anteriorly near their lower extremities, no doubt for the purpose of enabling the animal to roll itself into a ball. The tail is tapering, and is composed of ten articulations. The crustaceous covering is here more thickly deposited than on any other part. The entire shell seems to have been covered with minute elevated dots; these are beautifully distinct on the buckler and on the tail. Whole length of the specimen described, not quite two inches.