TEETH 



241 



lower end with an opening leading into the central pulp-cavity 

 (Fig. 191, B), into which blood-vessels and nerves extend. 



In most Vertebrates below Mammals all the teeth are essentially 

 similar in form (liomodont dentition} : in Mammals, on the other 

 hand, they become differentiated into distinct groups (heterodont 

 dentition}, known as incisors, canines, and cheek-teeth or grinders 

 (premolars and molars}. 



A succession of teeth takes place throughout life in almost all 

 Vertebrates except Mammals, in which there are very exceptionally 

 more than two functional sets, the so-called milk- or deciduous teeth 

 and the successional teeth. This difference is expressed by the 

 terms polyphyodont and diphyodont. (Comp. p. 245). 



Fishes, Dipnoans, and Amphibians. The homology and 

 similarity of the teeth with the dermal denticles of Elasmobranchs 

 has already been treated of (p. 30). The most primitive form of the 

 tooth is that of a simple cone, but even amongst Elasmobranchs, 

 in which the teeth are arranged in 

 numerous parallel rows upon the car- 

 tilaginous jaws, this form has already 

 become modified in various ways for 

 seizing or crushing the food. 



Of those Anamnia which possess 

 a bony skull, four groups of tooth- 

 bearing bones may in general be dis- 

 tinguished, viz., (1) the maxillary arch 

 (premaxiUa and maxilla] ; (2) the 

 palatal arch (vomer, palatine, ptery- 

 goid) ; (3) the (unpaired) parasphe- 

 noid : and (4) the rnandibular arch 

 (dentary and splenial). 1 



True teeth are wanting in Cyclo- 

 stomes, and amongst cartilaginous 

 Ganoids they are absent in the 

 adult Sturgeon, though rudiments 

 are present in the embryo. Amongst 

 Teleostei they are wanting in the 

 adult Lophobranchii and in Coregonus. In the Cyclostomes they 

 are represented functionally by a number of conical horny teeth. 2 



In bony Ganoids and Teleosts, teeth may be present on all the 

 bones bounding the oral cavity, as well as on the hyoid and the 

 branchial arches (" pharyngeal bones"). In the latter position, 

 as well as on the parasphenoid, they often form brush-like 

 groups. In form the teeth may be cylindrical, conical, or 

 hooked ; or they may be chisel-shaped (Scarus, Sargina9) , 



1 The teeth of Elasmobranchs may be compared to (2) and (4) of these. 



2 Structures bearing a superficial resemblance to vestigial true teeth are 

 recognisable beneath the horny teeth, but they possess no odontoblasts or 

 enamel epithelium. 



FIG. 192. SKULL OF Batrachowps 

 attenuatus. (From the ventral 

 side, showing the teeth on the 

 parasphenoid. ) 



