This fossil is among the number of fine specimens in the cabinet of J. P. Wetherill. It is in a rolled or contracted attitude, and is somewhat distorted. We have given, however, models of the head and the tail, in two distinct pieces. The external shell, or calcareous covering, is more perfect in this specimen than in any other we have ever seen. A considerable portion of the under side of the anterior part of the buckler, is also well preserved, and perfectly coincides with the figure and description given of it by Dr. Dekay and Mr. Stokes. There is another fragment of an Isotelus in the cabinet of Mr. Wetherill, showing eight articulations of the abdomen, which probably belongs to another individual of this species. The dorsal shell is in a high state of preservation. This species is embedded in clay slate, and was found in Newport, Kentucky. Genus Cryptolithus. _Green._ Among the numerous organic relics embedded in black limestone at Trenton Falls, in the State of New York, there is often found the fragment of a trilobite which cannot properly be referred to any of the genera already mentioned. Dr. J. Bigsby, in his Sketch of the Geology of the Island of Montreal, has figured and described a fossil which occurs at that place, which approaches in its specific characters to the fragments found at Trenton--but he does not suggest for his relic any name. Professor Brongniart has also represented, plate 4, figs. 5 and 7 A. B. C., the fragments of trilobites from Russia and from Llandillo, in Wales, which seem to differ but little from those above noticed, these are also without names. Under such circumstances, we have thought it expedient to group these relics under the generic term of Cryptolithus, a name analogous to Calymene, Asaphus, Ogygia, and Agnostus, and which may with propriety be applied to the animal, should it ever be discovered entire.[42] [Footnote 42: Since the above was written, and the C. Tessellatus published, I have received a fine specimen of this trilobite from Professor Eaton, in an almost perfect state, so that the entire animal can now be described.] _Body_, contractile. _Buckler_, lunate, convex, outer edge surrounded by a semicircular, reticulated, or tessellated border. _Front_ or middle lobe of the buckler very protuberant. _Oculiferous protuberances_, none. _Abdomen_, much compressed, trilobate. Cryptolithus Tessellatus. _Green._ Cast No. 28, and Fig. 4. Clypeo rotundato, fronte valde convexo, capite antice semicirculari margine tessellato ornato.