SURVEY OF MAMMALS 777 



The brain is poorly developed when compared with that 

 of other Mammals. Some of the characteristics of the 

 skeleton, etc., suggest Reptilian affinities. To this small 

 sub-class the titles Prototheria, Ornithodelphia, and Mono- 

 tremata 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 gestation, 

 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. IVIoreover, 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 Metatheria, Didelphia, 

 and Marsupialia are applied. 



C. In all the other Mammals there is a well-developed 

 allantoic placenta uniting the unborn young to the mother, 

 while in Marsupials this is only known in Perameles, where 

 it is of relatively little importance. It is among these 

 placental Mammals that the brain begins to be much con- 

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

 sub-class the titles Eutheria, Placentalia, and JMonodelphia 

 are applied. 



Among the extant orders of placental Mammals the Edentata and 

 the archaic Sirenia stand very much apart. The rest may be pro- 

 visionally grouped in three sets, perhaps representing three main lines 

 of evolution. 



On one side we place the great series of hoofed animals or Ungulates, 

 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 

 not far from the Ungulates it seems legitimate to rank (a) the whales and 

 dolphins (Cetacea), and {b) the rabbits and hares, rats and mice, etc. 

 (Rodentia). 



On the other side we place the great series of Carnivora, such as cats, 

 dogs, bears, and seals. Beside these may be ranked the Insectivora, 

 such as hedgehog, mole, and shrew, and the bats or Chiroptera, which 

 seem to be specialised Insectivores. 



In the middle we place the series which, beginning with the Lemurs, 

 leads through various grades of monkeys to a cUmax in man. 



But it must be carefully noted that these orders are often linked by 



