The other specimen of this species in the cabinet of the Albany Institute, is a large caudal end, three inches and a half long, entirely perfect. Both of these fossils were brought from Williamsville, Niagara county, New York. They occur in a dark shelly limestone, filled with other petrifactions. The calcareous matter which has mineralized the trilobite, in this instance, as in most others, is of a much darker hue than the surrounding rock. Genus Ceraurus. _Green._ _Body_, very much depressed, and slightly tapering. _Buckler_, scarcely trilobate; cheeks large, flat, with small remote oculiform tubercles; posterior angle of the buckler spinous. _Abdomen_, with twelve articulations. _Tail_, rounded at the end, but terminating on each side with two slightly curved spines. The name of this genus is derived from the remarkable spinous projections from the caudal end; this peculiar organization separates it widely from the other genera. The _Paradoxides Spinulosus_ of Wahlenberg, which is supposed to be the old _Entomolithus Paradoxus_ of Linné, the fossil, with which all the trilobites were for a long time confounded, has not only projecting spines from the tail, but from all the costal arches of the lateral lobes. The presence of eyes or of oculiferous tubercles in the _Ceraurus_, would alone be sufficient to separate it from the genus to which that interesting fossil belongs. In the eighth volume of Annales des Sciences Naturelles, Count Rasoumowsky has figured and described the fragment of a very curious relic, which seems to be an intermediate link between our genus and paradoxides; in addition to a number of filamentous elongations of the costal arches, a curved spine seems to project from the end of the tail, as in the _A. limulurus_. No name is given to this trilobite, which appears to have been found on the banks of the Yaousa, near Moscow, where it occurs in black, coarse, argillaceous schistus. The Ceraurus is probably a very rare animal remain, as we have only met with it, in the unrivalled cabinet of trilobites belonging to the Albany Institute. Ceraurus Pleurexanthemus. _Green._ Cast No. 33. Fig. 10. Clypeo postice arcuato, angulo externo in mucronem valde producto; oculis minimis remotis, postabdomine in spinam arcuatam acutam utrinque extenso.