242 Cincinnati Society of Natural History. 



residue, the ash as it were of the calcareous organisms." And as to 

 the rest they are sparsely distributed. In this respect the comparison 

 with the Triassic and Jurassic is most favorable, as the rarity of fos- 

 sils in the hands of the collectors very clearly testifies. 



But when we examine the fossils that have been discovered, and 

 note the evolution of forms, and compare these with the progress in 

 other ages, we are most pi-ofoundly impressed with the immense lapse of 

 time that must be ascribed to these periods. As not a single species 

 that is found in rocks earlier than the Triassic, and not one that is 

 found in rocks more recent than the Jurassic, has ever been found in 

 either the Triassic or Jurassic strata, we are sent at once to the genera 

 for comparison. Let us first turn to the Vegetable Kingdom. 



It is represented in the Triassic and Jurassic of North America by 

 66 described species, distributed among 30 genera. Twelve of these 

 genera are also of palaeozoic age, viz.: Calaviites, Chondrites, Cyclop- 

 teris, Dadoxylon, Fucoides, JSTeuropteris, Noeggerathia, Odontopteris, 

 Pecopteris., Sphenopteris, Toeniopteris., and Walchia ; and seven 

 genera are found in the Cretaceous, or more recent strata, viz. : Chon- 

 drites., Equisetum, Neuropteris, Fecopteris, Fterophyllum, Sphenop- 

 teris and Toiniopteris. This shows that five genera only, or one sixth 

 of all that are known, passed through this period, and that during this 

 period 16 genera, or more than half of what are known, came into ex- 

 istence, and also became extinct. The change of forms, as thus indi- 

 cated, is greater than that which has occurred to the Cretaceous flora 

 during all the ages that have elapsed to the present time. 



The evidence furnished by the invertebrate kingdom is no less strik- 

 ing. 



Thus far no species belonging to the Annelida or Crustacea, has 

 been described from these rocks, and the only articulated animal- 

 found fossil, so far as I have ascertained, is the 3Iorm ohicoides arti- 

 culatus, described by Prof. Hitchcock, in 1858, — a genus unknown in 

 other rocks. The class Pteropoda and the Rudista are unknown. The 

 class Polypi is not represented by a described species, and the Cavea 

 prisca alone represents the Bryozoa — another genus unknown in other 

 rocks. 



The Echiuodermata is represented by an Asterias and a Fentaoi- 

 nus, genera unknown in the Palaeozoic age, but one of them passed up 

 into the Cretaceous, and the other into the Tertiary period. 



The Brachiopoda are represented by eleven species belonging to the 

 genera Lingida, Fhynchonella, Spirifera and Tcrehratula. All of 

 these are Palaeozoic genera, and all of them have continued an exis- 



