432 THE GENERAL PRINCIPLES OF ZOOLOGY 



mated to cover about three million years, and we shall find the 

 fossils which it contains rapidly approaching the structure of 

 animals living to-day. In the Tertiary, with its three periods, 

 Eocene, Miocene, and Pliocene, we find not only existing fami- 

 lies of the invertebrates, but even many existing genera; yet even 

 in the Pliocene the species are for the most part different from 

 those of to-day, having become extinct and been replaced by 

 modern species. The Bryozoa or Polyzoa, which date back to 

 the Cambrian, were very abundant in certain regions during the 

 Tertiary, where their remains form calcareous deposits of very 

 great extent. The allied Brachiopoda, however, show a gradual 

 but sure decrease in numbers; thus there are 106 genera in 

 the Palaeozoic formations, 34 in the Mesozoic, and only 21 in 

 the Cenozoic. All the living orders of the Arachnida were 

 present in the Tertiary, although some had already long ex- 

 isted ; thus the scorpions date back to the Silurian, the spiders 

 to the Carboniferous. 



Of the Tertiary vertebrates we find that all the fishes belong 

 to existing orders. The curious amphibian Stegocephali dis- 

 appeared early in the Jurassic period of the Mesozoic, and the 

 first remains of the Urodelaand the Anura occur in the Eocene. 

 All the reptiles belong to existing orders, but some were much 

 larger than at present, as, for example, the gigantic land tortoise 

 of the Miocene in India, which measured six meters in length 

 and over two meters in height. Many existing orders of birds 

 appear in the Eocene, and several which have since become 

 extinct ; by the middle of the Tertiary nearly all of the existing 

 orders are represented. It is in this era that the Mammalia 

 become most conspicuous and attain their highest degree of de- 

 velopment. There are no remains of the Monotremata in the 

 Tertiary rocks. The Tertiary Marsupialia are found in Europe, 

 North America, and South America, although to-day there are 

 no representatives of this group in Europe. All of the existing 

 orders of the Placentalia are represented by Tertiary fossils, 

 and all appeared as early as the Eocene, with the exception of 

 the Edentata, which are found in the Miocene, although they 

 may have lived in the Eocene as well. Some suborders also, 

 such as the Proboscidea and the Anthropoidea, did not appear 

 until the Miocene. In addition there are several large groups, 



