130 THE ELASMOBRANCH FISHES 



resulting from a single narrow papilla, of which several may fuse at the base 

 into a simple triconodont tooth as in Chlamydoselachus (fig. 126a) ; or in the 

 rays a most complex arrangement of large median and smaller lateral hexag- 

 onal plates as in Myliohatis (fig. 126b) may result in pavement or crushing 

 teeth; or the plates may reach from side to side in an unbroken line as in 

 Aeiobatis. The teeth of Heterdontus (fig. 128) are particularly interesting in 

 that they represent both anterior prehensile and posterior crushing teeth. 

 Furthermore, in the area between these two, transitional teeth are present 





Fig. 128. Median view of teeth in jaws, Heterodontus francisci. 

 (Duncan Dunning, del.) 



which have characteristics of both. The teeth in the sixth row from the front 

 are flattened out and are provided with cusps essentially like the pavement 

 teeth of Miisielus henlei. 



The finer structure of a tooth (fig. 129) is somewhat similar to that of a 

 scale. In both there is an outside harder cap usually of structureless enamel 

 (e.) over a heavier inside layer of dentine. The enamel in some teeth is of a 

 coarse gi"ain, closely resembling dentine and by some held to be identical with 

 it. The dentine (d.) is of the tubular type formed by the odontoblasts of the 

 dentinal papilla. In the connective tissue cells at the base of the tooth vaso- 

 dentine [vd.) is formed, which is somewhat softer than dentine proper. In the 

 adult Myliohatis the whole core of the tooth becomes filled with dentine as 

 does the core in the tooth of the saw (Pristis), leaving only central canals from 

 which run numerous dentinal canals (d.c). 



It is generally held that enamel results from the basal layer of the epidermis, 

 with the exception that in Carcharias, according to Tomes (1898) it arises 

 from a special amelioblastic layer formed by the corium. All agree that den- 

 tine is produced in and by the corium. Here the odontoblasts send out pro- 

 cesses along which are deposited the lime salts which harden as dentine. The 

 processes become surrounded and are retained within dentinal canals {d.c, 

 fig. 129) which ramify throughout the dentine. In some forms the dentinal 



