FAUNA OF THE GEOLOGICAL PERIODS. 15 



and with lizard's tail, belonging to this period (Odon- 

 tornis Archceopteryx). Finally, it was during this period 

 that there appeared upon the scene that most perfect and, 

 for us, most important vertebrate class, the mammalian 

 class. The oldest fossil remains of these have been found 

 in the most recent Triassic strata, viz., molar teeth of 

 a small insectivorous Pouched Animal (Marsupial). Numer- 

 ous remains occur somewhat later in the Jura system, and 

 a few in the chalk. All the remains of Mammals from this 

 Mesolithic Epoch with which we are acquainted belong to 

 the low Pouched Animal division ; and among these were 

 undoubtedly the ancestors of Man. On the other hand, 

 not a single undisputed relic has yet been discovered 

 throughout all this period of one of the higher Mammals 

 (Placentalia). This last division, of which Man is a member, 

 did not develop till later, in the immediately subsequent 

 Tertiary Epoch. 



The fourth main division of the history of the organic 

 earth, the Tertiary, Osenozoic, or Csenolithic Epoch, was of 

 much shorter duration than the preceding. For the strata 

 deposited during this period are in all only about 8000 feet 

 in thickness. This Epoch, also, is divided into three sub- 

 divisions, known as the Eocene, Miocene, and Pliocene 

 Periods. During these periods the most diverse develop- 

 ment of the higher classes of plants and animals took place 

 and the fauna and flora of our globe now approached 

 nearer and nearer to their present character. The most 

 highly developed class of animals, that of Mammals, now 

 attained pre-eminence. This Tertiary Epoch may, therefore, 

 be called the age of Mammals. The most perfect section 

 of this class, the Placenta! Animals, among which is Man, 



