214 



>MPARATIVK ANATOMY. 



become differentiate. I into incisors, canines, and grinders 

 (premolars and molars). 



Fishes and Amphibia. The dermal denticles already de- 

 scribed in tlir chapter on the skeleton are structures homologous 

 \\ith teeth, t'.>r both are developed in a similar manner. In 

 'IVlrostei all the bones bounding the mouth may bear teeth, as 

 may also the hyoid and branchial arches (pharyngeal bones). On 

 the latter, and also on the parasphenoid, they are arranged in a 

 single series, or in masses ; numerous teeth are also met with on 

 the" parasphenoid in certain Urodeles (Fig. 174). In general, 

 however, there is in Amphibia a considerable diminution in the 

 number of teeth as compared with those of Fishes ; and at the 

 s:une time a much more uniform character is noticeable in their 

 form throughout. 1 



In Amphibia they are enlarged conically below, and rest on a 

 definite base, while above they become narrower, and slightly 

 curved, and end either in a double (Myctodera, Anura), or a single 



Kx:. 17k SKn.i, up Batrachoseps attenuatus. (From the ventral side, showing the 



teeth on the parasphenoid.) 



apex (Perennibranchiata, Derotremata, Gymnophiona) ; the latter 

 is the more primitive condition. The teeth lie deeply embedded 

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

 maxilla, maxilla, and mandible, as well as on the vomer and palatine, 

 but rarely on the parasphenoid ; in the larvas of Salamanders and 

 in Proteus the splennl of (he lower jaw is also toothed. Horny 

 jaws and horny teeth are present in larval Anura. 2 



1 In Fishes the teeth may 1m cylindrical, conical, or hooked ; or in some cases 

 (Scams and tin- Sarginse) they may be chisel-shaped, resembling the incisors of 



Mammals, and working together like scissors ; in others they give rise to a definite 

 iit, are rounded in form, and serve to crush the food. Again, they may he 



and 1 n 1st le-sha ] MM 1 M li ;> I o d o n), or sahro-slmped (Chauliodns). 



" The Imniv structures on the upper jaw in young stages of Polypterus, in the 

 iniiutli <if Cy.'lo-ti.n.i, mi the jaws of Siren lacertina, and on the lips of Dipnoi, belong 



to the same eati (>\ V as these. 



