8 



THE MAMMALIA. 



appearance of two sets of teeth in succession — the 

 milk and the permanent teeth. 



Milk and Permanent Teeth: Monophyodonts 

 and Diphyodonts.— The jaws of embryos and 

 young animals are relatively very short; but after- 

 warils they often become immoderately long. The 

 consequence of this elongation is that the first 

 teeth, which sometimes appear before, but mostly 

 not till after, birth, cannot occupy the jaws through- 

 out their whole extent A second set of teeth is 

 accordingly developed, altogether independently of 

 the first, and through the appearance of this second 

 set the number of the back teeth is in general 

 increased, while those in front are more or less 

 completely replaced by new and better arranged 

 teeth. Whatever be the mode in which this ex- 

 change is effected, and whatever processes nature 

 may resort to in attaining this end, we must before 

 all things keep in view the very considerable altera- 

 tions that are thereby brought about. 



First, it is manifest that this exchange is neces- 

 sary only where the teeth have a limited period 

 of growth. The teeth which keep constantly grow- 

 ing have no need to be renewed, though their 

 number or their size may have to be increased in 

 proportion to the growth of the jaws. The Cetacea 

 and the Edentata, in which no exchange whatever 

 takes place, have been distinguished as Monophy- 

 odonts, the others, which have two sets of teeth 

 in succession, as Diphyodonts. 



Yet here also there are essential differences. 

 The last stage of development, which is attained 

 by the apes and monkeys and the carnivores, is 

 very simple. All the front teeth, incisors, canines, 

 and premolars, which form the milk dentition of 

 a child of about seven years old, are deciduous, and 

 are replaced one by one by new teeth, while a few 

 molars are added in the back part of the jaws. 

 In such cases the distinction between the milk and 

 the permanent dentition will always be pretty easy. 

 More frequently, however, these relations are very 

 far from being so simple. 



Certain teeth, both of the first and second set, 

 develop indeed to a certain stage, but never cut the 

 gum. They remain entirely embedded in the gum 

 and arc afterwards absorbed. Others drop out 

 shortly after they have appeared, and if they belong 

 to the milk dentition arc not replaced by permanent 

 teeth. Finally, the number of the teeth which are 

 exchanged is also very variable. Thus in the 

 marsupials only a single tooth, the last premolar, 

 is shed, while all the others remain in their original 



places. The order in which the teeth arc replaced 

 is just as variable as the time at which this phe- 

 nomenon occurs. Some bats, for instance, lose 

 their first teeth while still in the womb of the 

 mother. In many cases, moreover, the first set 

 of teeth is very different from the second, and, 

 indeed, often has a considerably different character. 

 The aye-aye is an insectivore in its milk dentition, 

 but in its permanent dentition a rodent. And if 

 the entire dentition does not often alter its character, 

 this is frequently the case with at least a certain 

 number of the teeth. The carnassial tooth of the 

 Carnivora alters its place. It travels, so to speak, 

 towards the back of the mouth in the permanent 

 dentition. 



The general fact remains, however, that a certain 

 number of back teeth are never shed, and only 

 appear once for all, namely, at the time when the 

 shedding of the other teeth is in progress or nearly 

 completed. These teeth commonly have a more 

 complicated form than the others. They are those 

 which are called true molars. 



But let us return to the teeth themselves, and 

 consider first their form and structure. If the form 

 of a simple cone, a cylinder, or a column is the 

 primitive one, the form which is still retained most 

 frequently in the front teeth, it cannot be asserted 

 nevertheless that it is the form universally retained. 

 It may be materially modified by the presence of 

 notches and folds, clothed with enamel, and situated 

 either on the grinding surface or on the sides of the 

 crown. If such folds are found on the grinding 

 surface, then there arise grooves and depressions, 

 between which higher parts remain standing in the 

 form of tubercles, ridges, and peaks. All these 

 structures may be reduced to three leading types. 

 In one the tubercles get worn down to a level 

 surface and remain low; they are broad and rounded, 

 the crown likevvi.se becomes flat and more or less 

 quadrangular. This is the type of the omnivorous 

 tooth, such as we find in pigs, bears, and others. 

 Secondly, the tubercles may be pointed and get 

 worn away on the lateral surfaces by friction against 

 the teeth in the opposite jaw. In this manner are 

 formed the teeth with pointed cusps of the insect- 

 eaters. Thirdly, the tubercles may be elongated 

 in the direction of the jaw into sharp lobes, which 

 act like shears against the corresponding lobes of 

 the opposite teeth. Of this type the teeth of the 

 carnivores offer striking examples. 



The lateral folds of the crown, which in the first 

 instance are mere vertical flutings, may become 



