?AL;EZOOLOGY : FOSSIL ANIMALS. 263 



we have just seen, are those of certain fishes ; ascending 

 in the scale of superposition, we find first reptiles, and 

 then mammalia. The first reptile (a Saurian of the genus 

 Monitor, according to Cuvier), and which had already at- 

 tracted the notice of Leibnitz, ( 303 ) is found in the copper 

 slate (Kupferschiefer) of the zechstein in Thuriugia; the 

 Paleosaurus and the Thecodontosaurus of Bristol, belong, 

 according to Murchison, to the same period. The number 

 of Saurians continues to augment in the muschelkalk,( 304 ) 

 tlie keuper sandstone, and the oolite in which they reach 

 their maximum. At the oolitic period (including under 

 this name the lias) lived several species of Plesiosaurus, an 

 animal having a long swan-like neck, formed in some cases 

 df upwards of thirty vertebrae ; the Megalosaurus, a gigantic 

 reptile of forty- five feet in length, with bones of the extre- 

 mities resembling those of the heavy terrestrial quadrupeds; 

 eight species of Ichthyosauri with enormous eyes; the 

 Geosaurus (Sommering's Lacerta gigantea) ; and seven species 

 ctf hideous Pterodactyles, or reptiles with membranous 

 wings. ( 305 ) There existed also towards the latter part of 

 the period the colossal Iguanodon, an herbivorous animal , 

 and in the chalk where the number of crocodilian Saurians 

 begins to diminish, we find the Mososaurus of Conybeare, 

 a crocodile of Maestricht. We learn from Cuvier, that 

 animals belonging to the present race of crocodiles are found 

 in the tertiary formations ; and Scheuchzer's supposed hu- 

 man skeleton (homo diluvii testis), a great salamander 

 allied to the axolotl which I brought from the lakes around 

 the city of Mexico, belongs to the most recent fresh water 

 formations of (Eningen. 



In studying the relative age of fossils by the order of 

 superposition of the strata in which they are found, impor- 



