GENERAL SURVEY OF MAMMALS. 633 



eaters {Echidna and Proechidna) differ markedly from other 

 Mammals. The young are hatched outside of the body ; 

 in other words, the mothers are oviparous. The brain is 

 ^ poorly developed when compared with that of other Mam- 

 mals. Some of the characteristics of the skeleton, &c., sug- 

 gest Reptilian affinities. To this small sub-class, the titles 

 Monotremata, Ornithodelphia, and Prototheria are applied. 



{B) The kangaroos and bandicoots, phalangers and 

 opossums, and the like, form the second sub-class. In 

 these the young are born prematurely after a short gesta- 

 tion, during which the organic connection between the 

 mother and the young is comparatively slight. Most 

 female Marsupials have an external pouch or marsupium, 

 to which the tender young are transferred, and within 

 which they are nourished and protected for some time. 

 Moreover, the brains even of the most intelligent Marsupials 

 are not so well developed as those of higher Mammals. 

 To this heterogeneous sub-class, the titles Marsupialia, 

 Didelphia, and Metatheria are applied. 



(C) In all the other Mammals there is a placenta uniting 

 the unborn young to the mother. It is among these 

 placental Mammals that the brains begin to be much con- 

 voluted, — as it were, wrinkled with thought. To this sub- 

 class, including sloths and ant-eaters (Edentata), sea-cows 

 (Sirenia), hoofed-animals (Ungulata), Cetaceans, Rodents, 

 Carnivores, Insectivores, Bats, Lemurs, and Monkeys, the 

 titles Placentalia, Monodelphia, and Eutheria are applied. 



Among these orders of placental Mammals, it seems 

 likely that the Edentata and Sirenia should be placed lowest, 

 for many of their characteristics are old-fashioned. The 

 rest may be provisionally grouped in three sets, perhaps 

 representing three main lines of evolution. 



On one side we place the great series of hoofed animals 

 or Ungulata, including (a) those with an even number of 

 toes (Artiodactyla), such as pigs, hippopotamus, camels, 

 cattle, and deer ; {b) those with an odd number of toes 

 (Perissodactyla), such as tapir, rhinoceros, and horse ; {c) 

 the elephants (Proboscidea) ; (d) the Hyraxes (Hyracoidea). 

 And near the Ungulata it seems legitimate to rank (a) the 

 whales and dolphins (Cetacea), and (/;) the rabbits and hares, 

 rats and mice, &c. (Rodentia). 



