S8 THE ANATOMY OP VEETEBRATED ANIMALS. 



structures, liomologous witli 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, characterised 

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

 branching as they go, constitutes the chief mass of tx-ue 

 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, prismaticaUy fibrous, 

 ena'inel. 



The teeth are moulded upon papillse 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 

 either as diverticula of the old ones, or independently of 

 them. 



In the majority of the Mamnudia the teeth are limited in 

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

 sixccession. There are two sets of teeth, forming a first, deci- 

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

 The deciduous dentition, when most completely developed, 

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

 distinguished from the rest by the lodgment of the upper set 

 in the premaxillse, and the correspondence of the lower set 

 with the upper. Their number and form vary. The dis- 

 tinction 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 formula is a convenient 

 combination of letters and figures for making the number 

 and disposition of the teeth obvious. Thus, let di, dc, dm 

 represent, respectively, 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 



