XIII MAMMALIA 439 



bears a rough relation to the number of young produced at one birth by 

 the particular species of animal. 



The epidermis of the mammal is underlaid by a dermis containing 

 a great development of strong fibres running in all directions which give 

 it the peculiar toughness characteristic of leather. 



Such are the main features characteristic of the skin of mammals. 

 There occur very many special developments of interest. Horny epi- 

 dermal scales may cover the general surface of the body (Manis) or parts 

 of it (tail of Rat). The claws may be modified as hoofs or nails. The 

 bony horns of the Ruminants, such as the Cow, Sheep and Goat, are 

 encased in a special horny sheath. The horn of the Rhinoceros, on the 

 other hand, is horny and epidermal throughout. The transverse ridges 

 across the palate grow out in one of the groups of Whales into flat plates 

 of horn (whale-bone) which fray out at their edges into a fibrous mass 

 that serves as a strainer to strain off from the sea-water the minute 

 animals (plankton) upon which the whale subsists. 



In the alimentary canal of the mammal one of the most conspicuous 

 characteristics is the high degree of specialization of the teeth. It will 

 be remembered that in the Dogfish there are arrangements for replacing 

 the teeth as they become worn out, there being, so to say, numerous , 

 generations of teeth developed one after the other. Now it is highly 

 characteristic of the mammal that a single set of teeth — ^termed the 

 permanent dentition — remain functional during the greater part of the 

 life of the individual. The immediately preceding generation is as a 

 rule represented by a set of functional teeth present for a short period 

 in the young animal and known as the milk dentition. The numerous 

 other dentitions have been completely eliminated as functional organs, 

 although in various mammals recognizable microscopic vestiges of a 

 " pre-milk " and a " post-permanent " dentition have been found. 



This reduction in the succession of the teeth has been accompanied 

 by great advances in their individual structure. Whereas in the lower 

 vertebrates the teeth are as a rule similar in form and structure (homodont) 

 they are in the typical mammal specialized for different functions, those 

 in front being flattened chisel-like cutting teeth (incisors), the next being 

 conical pointed tearing teeth (canines), and the hinder ones being grinders. 

 The more anteriorly placed grinders, usually distinguishable from the 

 hinder ones by their simpler form and by the fact that each of them is 

 preceded by a milk-grinder, are termed pre-molars, while the hinder ones, 

 more complex in form and without milk predecessors, are termed molars. 



The greatest complexity in form occurs in the grinder teeth. The 

 crown of the tooth — the part projecting from the jaw — has its surface 



