x. 
attacks of enemies. On the Pangolins (Manide) of Africa and 
southern Asia alone, true imbricated scales, for the protection of 
the body, are present; between which, however, isolated hairs 
occur. Similarly imbricated epidermal productions are also found 
on the tails of many Rodents, such as the Beaver and the Rat ; 
of some Insectivores, such as Myogale ; and of some Marsupials, 
such as the Didelphyide ; among these Groups, however, these 
scales are frequently confined to the under surface of the tail. 
Only the Armadillos of South America possess a true bony exo- 
skeleton (somewhat similar to that of the Tortoises), which is 
covered with scutes of horny epiderm, and which is eminently 
fitted for defence against all ordinary enemies. The horns of the 
Ruminants and Rhinoceroses are also modified forms of epidermis, 
as also are nails, claws, and hoofs, and the perforated spur of the 
Monotremes. 
DENTAL SYSTEM. 
Dentition.—In all mammals, except the Narwhal, the teeth of 
the opposite sides of each jaw are alike in number and character. 
There are two distinct forms of dentition, the Homodont and the 
Heterodont ; the former, of which the Dolphins are the best 
exemplars, being, as its name implies, the more simple; in these 
the crowns are single-pointed and slightly curved, the roots also 
single and tapering, and all of similar formation, those in the 
middle of the series being, however, as a rule longer than those at 
either extremity. In the Heterodont dentition, on the contrary, 
the teeth are of different forms; the front teeth, or Incisors, are 
simple and one-rooted, and are adapted for cutting and seizing, 
while the back teeth, or Molars, have tuberculated or ridged 
crowns, are supported by two or more roots, and are specially 
formed for crushing and grinding the food ; between these two 
series there is frequently a longer and more sharply pointed tooth, 
popularly known as the “‘eye-tooth,” and technically—on aceount 
of its having attained to its highest development in the Wolves and 
their allies (Canide )—termed the Canine. By a similar process 
of development, though under widely different circumstances, the 
Marsupial Wolf or Tiger of Tasmania (Thylacinus/ has arrived at 
a precisely similar tooth ; the use of the canines is principally 
the tearing of the flesh of their victims and the holding of struggling 
prey. The molariform series is divisible into two parts, such of the 
posterior teeth as are without milk-predecessors being termed 
* Molars,” such as have milk-predecessors ‘“ Premolars.” In the 
Eutherian Mammals this series is generally constituted of four 
premolars and three molars, with a milk dentition normally con- 
sisting of three, the last premolar having invariably a predecessor ; 
but among the Marsupials this order is reversed in the permanent 
teeth, the number of premolars being one less and of molars one 
more, while the milk dentition, if present, is limited to a single 
