GENERAL CHARACTERS. 893 



third constituent, usually known as cement (fig. 813, e), but occasion- 

 ally as crusta petrosa, is the most external of the three : in some 

 instances, as in fig. 813, it is confined to the root, or embedded 

 portion of the tooth, where it consequently comes into immediate 

 contact with the dentine ; but in other cases it is found overlying 

 the enamel of the crown, and in others again, where the enamel is 

 wanting, it forms the main covering of the crown, as in the teeth 

 of the Edentate Mammals. The cement is always traversed by 

 vascular canals ; and its structure is very similar to that of bone, 

 although only occasionally presenting all the peculiarities of the 

 latter. 



True teeth are generally confined to the mouth and pharynx, but 

 they may be situated on many of the bones of the former, and their 

 mode of attachment varies from a simple anchylosis to the under- 

 lying bone, to implantation in distinct sockets. The simplest forms 

 of teeth are small granular bodies, like the minute ossifications 

 already mentioned as occurring in the skin of certain Fishes ; while 

 those of the most complex structure are to be found in certain Fishes 

 (Dendrodus), in the Labyrinthodont Amphibians, and among Mam- 

 mals in the Edentate genus Oryderofius and many Rodents and 

 Ungulates. Except in forms where all are alike, as a general 

 rule the teeth at the anterior extremity of the jaws are more or 

 less simple, while there is a gradual increase in their complexity 

 towards the opposite end, and in most cases the lower teeth are 

 narrower and more elongated in an antero-posterior direction than 

 the upper ones. 



The importance of teeth to the palaeontologist as a means of 

 determining the affinities of fossil forms has been already mentioned ; 

 but in many cases their evidence must be supplemented either by 

 that of other remains, or of the geological horizon whence they were 

 obtained ; since otherwise serious error may result. Thus, the teeth 

 of the Dinosaurian Megalosaurus present a strong resemblance to 

 some of those of the Mammalian Machcerodus ; while the front teeth 

 of some Sparoid Fish approximate to those of the Primates ; and the 

 lower hinder teeth of the Kangaroos, of the Dinothere among the 

 Proboscidea, and of the Tapir-like animals in the Perissodactyla, are 

 all singularly alike in form. Another fertile source of error to be 

 guarded against is the great difference in the form of the teeth from 

 different regions of the mouth. 



For the microscopic structure of teeth and their mode of develop- 

 ment, the student must refer to other works ; but a large number 

 of the more important types of dental structure will be found noticed 

 in the course of the following pages. 



By far the greater number of adult Vertebrates possess a solid 

 inner, or endoskeleton, composed of bone (into the nature of which 



