X. 



attacks of enemies. On the Pangolins (Manidce) 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 Didelphyidie ; 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 

 Monotrernes. 



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 

 Heteroclont ; 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 account 

 of its having attained to its highest development in the Wolves and 

 their allies (Canidw) termed the Canine. By a similar process 

 of development, though under widely different circumstances, the 

 Marsupial Wolf or Tiger of Tasmania (ThylacimisJ 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 



