INTRODUCTION. 



not only does the division of the Mammalia into 

 subordinate groups depend in a great measure on 

 the dental system, but this system also possesses 

 the great value of being the essentially conservative 

 element of the whole skeleton. If all the variations 

 in the surroundings, in the nature of the food, and 

 the mode of life are reflected in the structure of 

 the teeth, there is, in fact, no other part of the 

 skeleton accessible to the palaeontologist which 

 preserves the essential characters of a type with 

 such certainty, and in this way enables him to 

 recognize the points of resemblance and affinity 

 that may obtain among the different forms. In 

 any case the study of the dental system forms the 

 foundation of a knowledge of the Mammalia 

 generally. 



Only an infinitesimally small number of mammals 

 are actually without teeth. The porcupine ant- 

 eaters, the ant-bears, the scaly ant-eaters, and the 

 whalebone whales belong to the number. Yet 

 teeth have been found in great numbers in the 

 jaws of embryo whales, but these teeth are com- 

 pletely embedded in the gum, which they never 

 cut, and are afterwards re-absorbed when the 

 whalebone has formed in the mouth. It is likewise 

 highly probable that traces of teeth will yet be 

 found in the jaws of the embryos of the terrestrial 

 mammals mentioned. Analogy seems to prove 

 that the absence of teeth in mammals is always 

 the result of a process of reduction carried to the 

 last extreme. 



Structure of the Teeth. The teeth of mam- 

 mals are formed in closed sacs or depressions in 

 the jaw, and do not cut the gum till they have 

 reached an advanced stage of development. The 

 proper nucleus of a tooth is composed of the so- 

 called dentine. This peculiar bony substance is 

 characterized by the presence of numerous branch- 

 ing canals (tubuli) which run from the inner cavity 

 of the tooth (the pulp cavity) towards the surface. 

 Whatever the subsequent form of the tooth may 

 be, this dentine is always originally deposited as 

 a sort of cap round a fleshy protuberance rich in 

 vessels and nerves, and the hollow of this cap is 

 filled with dentine. 



On the latter there rests in most cases a second 

 cap composed of very hard columns set close to- 

 gether, and forming what we call the enamel. This 

 shining brittle substance usually covers only what 

 is called the crown of the tooth, that is, the part 

 that rises above the gum. For the most part the 

 crown is protected on all sides by this covering; 



but in many cases, as, for example, in the incisors 

 of rodents, we find it mainly on the front of the 

 crown, and only a very thin plate behind. The 

 enamel thus extends in general as far as the gum, 

 and thus serves to distinguish the crown from the 

 root, which is set in the gum. Besides these two 

 chief substances, there are frequently also other 

 tissues entering into the composition of a tooth 

 true bone-tissue at the root and cement on the 

 crown, this last substance being peculiarly abun- 

 dantly developed in compound teeth, in which it 

 fills up the folds and other depressions. 



In the structure of the roots or fangs considerable 

 diversities appear, in consequence of which we have 

 teeth that keep constantly growing and others 

 with a limited period of growth. Consider, for 

 instance, the incisor of a rodent. The pulp cavity 

 opens out wide at the lower end, where the dentine 

 gradually thins away, while the pulp nourishing 

 the tooth fills up the whole cavity. Teeth so 

 formed keep on growing through life, and this 

 constant growth is counteracted only by the fact 

 that the crown gets constantly worn away by use. 

 The tusks of the elephant and those of the wild 

 boar are both of this nature. In a number of other 

 teeth, however, the pulp cavity gets gradually 

 narrower towards the bottom, where it forms at 

 last only a minute opening. This condition is 

 brought about in the course of the development 

 of the tooth, for at first the root is always wide 

 open, and after this constriction of the pulp has 

 taken place the tooth ceases to grow, since the 

 vessels that ascend through it are only sufficient 

 for the nourishment of the tooth. Most of the 

 teeth of mammals have this structure. They may 

 be replaced by a later set pushing out the earlier 

 ones; but once formed they undergo no further 

 change in the way of growth. 



Teeth with several fangs are found only in 

 mammals. A tooth with a double or treble fang 

 belongs unquestionably to a mammal; and, indeed, 

 most molars and premolars have compound fangs, 

 while incisors and canines, with very few exceptions, 

 have only single roots. 



From our description it will be seen that the 

 more or less conical teeth with open roots, such as 

 we find, for example, in the dolphins, represent the 

 original form, a form which is also found in the 

 reptiles and amphibians. The closing of the root 

 and the cessation of growth mark a higher stage 

 of development, which leads to a phenomenon 

 occurring only in the Mammalia, namely, the 



