TEETH 



315 



are wanting in Cyclostomes, and are represented functionally by 

 a number of conical horny teeth, the morphological nature of 

 which has been variously interpreted. Amongst cartilaginious 

 Ganoids teeth are absent in the adult Sturgeon, though present 

 in the embryo. Amongst Teleostei they are wanting in the 

 Lophobranchii, and, except in very early stages, in Coregonus. 



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

 bones bounding the oral cavity, as well as on the hyoid arid 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, hooked, or chisel- 

 shaped (Scarus, Sarginae) ; in some forms they give rise to a 

 definite pavement, are rounded in 

 form, and serve to crush the food ; 

 in others, again, they are delicate and 

 bristle-like (Cha?todon), or sabre- 

 shaped (Chauliodus). 



In the Dipnoi (Fig. 71) the teeth, 

 which are wanting in enamel (though 

 an enamel organ is present in the 

 embryo Lepidosiren), are exceedingly 

 massive, presenting sharp edges and 

 points : they have probably arisen 

 by the concrescence of a number of 

 individual teeth. 1 More particularly 

 in Ceratodus, the origin of the vomer, 

 palatopterygoid, and dental plates of 

 the mandible from a fusion of the 

 bony basal portions of the teeth is FIG. 230. SKULL OF 

 very evident, and the same has been 

 shown in the case of the Amphibia. 



In the Amphibia there is in 

 general a considerable diminution in the number of teeth 

 as compared with Fishes ; and at the same time a much 

 more uniform character is noticeable in their form through- 

 out (Fig. 231. A, B). They are conical, enlarged below, and 

 rest on a definite base, while above they become narrower and 

 slightly curved, ending either in a double (Myctodera, Anura), or 

 a single apex (Perennibranchiata, Derotremata, Gymnophiona) ; the 

 latter is the more primitive condition. The teeth lie deeply em- 

 bedded in the mucous membrane, and are present, as a rule, on the 

 premaxilla, maxilla, and mandible (except in Anura), as well as on 

 the vomer and palatine, but rarely on the parasphenoid (e.g. 



1 There are 110 indications of a succession of teeth in the Dipnoi. In 

 Protopterus the teeth are covered by an epithelial horny layer during the torpid 

 period. The formation of complex teeth by concrescence is apparently not so 

 frequent amongst Vertebrates as was formerly supposed, and does not apply, e.g. 

 to those of various Elasmobranchs, of Labyrinthodonts, Ichthyosaurians, and 

 probably also to the mammalian molars. 



teeU, 



parasphenoid. 



