GENERAL SURVEY OF MAMMALS. 693 
of the body; in other words, the mothers are oviparous. 
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. 
8B. 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. 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 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 Monodelphia 
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 ; (4) 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 (4) 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 climax in man. 
