4:4 Cincinnati Society of Natural History. 



reach the Cretaceous era. Where tracks have been described new 

 genera in all instances have been proposed. 



The Mammalia are represented in the Triassic rocks by Dromathe- 

 rium silvestre, described b}^ Prof. Emmons, in 1857. Four genera 

 have been named from the Jurassic, viz: Ctenacodon, Dryolestes, 

 Stylacodon and Tinodon. These genera are not only confined to the 

 rocks in question, but they are not referred to families found in other 

 rocks. 



Or taken as a whole, the vertebrate kingdom is represented hy 57 

 genera, two of which only are referred to rocks of earlier date, and 

 only two to a later period. 



These calculations are based upon our present knowledge of the 

 fauna and flora, but as new discoveries are being made almost daily, 

 we can not tell how much the}' may be modified in future. It will be 

 observed, however, that an increased number of species will not change 

 the calculations, and that an increase of the genera is more likely by 

 adding new ones, than by the discovery of either Palaeozoic or Creta- 

 ceous genera in these rocks, 



AmphicoiUas fragillirmis was described by Prof. Cope, from near 

 Canyon Cit}^, Colorado, in 1878. (See ^m. Nat. for August.) Hypsir- 

 hopus seeleyanus should have been referred to the Jurassic of Wyom- 

 ing, instead of Colorado; and Paloioctomis appalachianus, Suchopriau 

 aulacodus, Clepsysaurus veatleianus, Suchoprion cypfwdon, Thecodon- 

 tosaufus gibbidens, and Palceosaurus frazeranus should have been re- 

 ferred to York County, Peunsj'lvania, instead of Phoenisville. 



We will now pass the Triassic and Jurassic periods for the purpose 

 of considering the Cretaceous or last period that is referred to Mesozoic 

 age. 



CRETACEOUS. 



The existence of the Cretaceous formation, upon this continent, was 

 first determined in the year 1827, when Dr. S. G-. Morton and Lardner 

 Vanuxem* compared the marl of New Jersey with the Cretaceous of 

 Europe, called by the 'French la craie inftrieure ou aucienne, and by 

 the English the Green Sand formation or Ferruginous Sand-series. 



In 1828, Dr. J. E. DeKayf described, from New Jersey, Ammonites 

 Jdppocrepis, now Scaphites hippocrepis., and A. placentcl.^ now Placen- 

 ticeras placenta. 



[To BE Continued.] 



* Am. Jour. Sci. & Arts, vol, 12. 



t Ann. Lye, Nat. Hist., N. Y., vol. 2. 



