q8o The Reptiles of tin Ameiiean Eocene. [December, 



most prolific of reptilian life. It is true that the orders of reptiles 

 which characterized the Mesozoic periods no longer existed. The 

 Dinosauria had perished from the land ; the Ichthyopterygia, Sau- 

 ropterygia and Pythonomorpha no longer inhabited the sea, and the 

 Pterosanria had disappeared from the air. What occasioned the 

 remarkable change in reptilian life at the close of the Laramie 

 epoch can only be surmised. During that time the principal land 

 population of North America consisted of Dinosauria, of which 

 there were many species and genera. With the opening of the 

 Puerco Eocene, these huge beasts had entirely disappeared, and 

 a population of small and medium sized Mammalia took their 

 place. The comparative feebleness of the new comers precludes 

 the idea that they assaulted and drove out or killed the Dinosau- 

 ria, or that they devoured their food and left them to starve. 

 The only probable hypothesis must suppose that a change of 

 climate ensued, either in a depression of the temperature, or in 

 a desiccation of the atmosphere, which greatly reduced the amount 

 of vegetable life. The large Dinosauria would perish from lack 

 of food, where smaller animals could live. That there was a gen- 

 eral desiccation at the beginning of the Eocene period in central 

 North America is indicated by topographical evidence. It was 

 towards the close of the Laramie that the elevation of the Rocky 

 mountains was completed, and their greatest effect in retaining 

 the clouds and rains, must have been apparent Nevertheless, 

 this effect could not have continued, since the later Eocene and 

 Miocene epochs were rich in forests and animal life. 



The Eocene reptiles were not a new creation, nor a new evolu- 

 tion, but a remnant of the types that had coexisted with the mon- 

 archs of life during previous ages. We must except from this 

 statement the serpents, which first appear in numbers at this time, 

 only one cretaceous species having been found by Dt. Sauvage, in 

 France. The crocodiles, tortoises, and lacertilians represent orders 

 already abundant in the Mesozoic faunae. Their decadence in Cen- 

 tral North America did not commence until the Miocene period, 

 when the crocodiles and nearly all the tortoises disappeared. F rom 

 the Loup Fork or Upper Miocene, only a few traces of lizards 

 have been obtained, and snakes were apparently not very numer- 

 ous. On the eastern coast regions, crocodiles existed, and tor- 

 toises were more numerous during the Miocene period ; but here 

 also they were less abundant and varied than during the Eocene. 



