MAMMALS IN GENERAL ii 



habit of hatching the egg within the mother's body and producing 

 living young. 



The mammals probably developed from a group of reptiles 

 generally known as the Anomodonts, and these again arose (in 

 common with the rest of the reptilia) from amphibian forms 

 alHed to the extinct Labyrinthodonts. The Anomodont or 

 Theriodont reptiles (remains of which are found all over the world, 

 but principally in North America, Europe, and Africa) differed 

 from other known reptiles in the differentiation of their teeth on 

 mammalian lines ; that is to say, the specialisation of prehensile 

 teeth answering to our incisors, of tusk-like canines, and of molars 

 and premolars that exhibited three or more cusps in the single 

 tooth. The first mammals were creatures allied in their structure 

 to the Monotremes still existing in Australia. From this stock 

 developed what is known as the Eutherian, or true mammalian 

 form. Among the most primitive of the true mammals were 

 the marsupials, who retain many low characteristics coupled with 

 a certain amount of special development and some degeneracy. 

 At the present day marsupials are limited in their distribution to 

 Australia, New Guinea and the southern islands of the Malay 

 Archipelago, and also to North and South America ^ ; but 

 anciently there were marsupials in Great Britain, creatures some- 

 what resembling the opossums of America and the banded 

 ant-eater of Australia. 



When the earth was passing through the first phase of the 

 Tertiary Epoch (the Eocene period), when in fact the British 

 Islands were beginning to assume something like their present 

 relations with Europe (instead of being an outlying part of North 

 America separated from Europe by a shallow sea), the true 

 mammalia were already spreading out into many orders, a few 

 of which are extinct, but most of which have their living repre- 

 sentatives at the present day. These existing orders are (i) the 

 Marsupials ; (2) the Edentates (armadillos, ant-eaters, sloths, etc.) ; 

 (3) the Whales ; (4) the Ungulates (all hoofed mammals) ; 

 (5) the Sirenia (manatis, dugongs, etc.) ; (6) the Carnivores 



^ They originated probably in North America. 



