The Cryptolithus Tessellatus occurs also in the limestone which, according to Dr. Bigsby, overlays the sandstone in the island of Montreal. At most of its localities, it is associated with the Isotelus, the Calymene, and with several species of Asaphus. The Cryptolithus, which is entirely destitute of eyes, being thus found with the oculiferous species, is an interesting fact, and controverts the opinion of Professor Wahlenberg, that the trilobites, which are without eyes belong to a geological epoch more ancient than those which are furnished with oculiform tubercles. That organic remains furnish us with the most satisfactory evidence of the identity or dissimilarity of certain formations, is a disputed point with some geologists[45] It cannot reasonably be doubted, that new and isolated facts have been made the basis of a too hasty generalization. On this subject Count Rasoumowsky makes the following remarks:--"Les divers gisemens des Trilobites ne me semblent pas non plus pouvoir être déterminés avec quelque précision. M. Brongniart parait admettre que les trilobites aveugles ne se trouvent que dans de tres anciennes formations dans des schistes et des calcaires de transitions; mais nous avons donné la description d'un trilobite [without eyes] des bords de la Yaousa prés de Moscow, qui n'appartient certainement pas aux formations de transition, ce qui me donné lieu de croire que de nouvelles recherches et de nouvelles observations, prouveront qu'il n'est pas strictement vrai qu'en France, en Angleterre, en Russie, _il n'existe point de trilobites entiérement privés d'yeux_, comme le dit le savant auteur que je viens de citer." See Annales des Sciences Naturelles, Vol. 8. page 195. [Footnote 45: See Eclectic Review, for July, 1832.] Genus Dipleura. _Green._ _Body_, contractile, not much depressed, and slightly tapering. _Buckler_, pustulous, trilobate, cheeks protuberant, with oblique, annular, oculiferous tubercles. _Abdomen_, with fourteen articulations, not lobate, the ribs double. _Tail_, suborbicular, not so large as the buckler, covered with an epidermis. This genus derives its name from two Greek words, which signify double ribs; many of the trilobites are thus characterized; but in no species, is this organization so remarkable as in those which belong to the Genus Dipleura. The expansion of the tail resembles, in some degree, that of the Isotelus, but other obvious characters sufficiently distinguish it from that interesting genus. The fossils arranged under this section are larger than most other trilobites. Dipleura Dekayi. _Green._ Casts No. 30, 31, and Figs. 8 & 9. Clypeo lunato punctato; abdomine quatuordecim articulis duplicibus vix lobatis; cauda suborbiculari; limbo lato convexo integerrimo; oculis oblique deflexis.