80 TUE ANATOMY OF VERTEBRATED ANIMALS. 



" ecderonic," cuticular, or epithelial structures. In Vertebrata 

 true teeth are invariably " enderonic," or developed, not from 

 the epithelium of the mucous membrane of the alimentary- 

 canal, but from a layer between this and the vascular deep 

 substance of the enderon, which answers to the dermis in the 

 integument. The horny " teeth " of the Lampreys, and of 

 Ornithorhynclms, appear to be ecderonic structures, homolo- 

 gous with the " baleen " of the Cetacea, with the palatal 

 plates of the Sirenia, or the beaks of Birds and Reptiles, and 

 not with true teeth. 



The dense calcified tissue called dentine, characterized by 

 the close-set parallel tubuli which radiate through it, branch- 

 ing as they go, constitutes the chief mass of true teeth ; but 

 the dentine may be coated with ordinary bony tissue, which 

 then receives the name of cementum, and its crown may be 

 capped with imperforate, prismatically fibrous, enamel. 



The teeth are moulded upon papillee of the mucous mem- 

 brane, which may be exposed, but are more usually sunk in a 

 fold or pit, the roof of which may close in so as to form a 

 dental sac. And there may be one set of teeth, or several ; 

 the sacs of the new teeth, in the latter case, being developed 

 cither as diverticula of the old ones, or independently of them. 



In the majority of the Mammalia the teeth are limited in 

 number, as well as definite in their forms and their mode of 

 succession. There are two sets of teeth, forming a first, decidu- 

 ous, or milk dentition, and a second, or permanent dentition. 

 The deciduous dentition, when most completely developed, con- 

 sists of incisor, canine, and molar teeth. The incisors are 

 distinguished from the rest by the lodgment of the upper set 

 in the premaxillsB, and the correspondence of the lower set 

 with the upper. Their number and forai vary. The distinc- 

 tion between canines and molars is one of form and position 

 in regard to the remaining teeth ; the most anterior of the 

 teeth behind the premaxillo-maxillary suture, if it is sharp and 

 projecting, receiving the name of canine. There are never 

 more than four canines. The other teeth are molars, and 

 ordinarily do not exceed four upon each side, above and below. 

 What is called a dental formida is a convenient combination 

 of letters and figures for making the number and disposition 

 of the teeth obvious. Thus, let di, dc, dm, represent, respec- 

 tively, the deciduous or milk set of incisors, canines, and 

 molars. Then, by placing after each of these symbols figures 

 arranged so as to show the number of the teeth of the kind 

 symbolized, on each side of each jaw, we shall have the dental 



