214 COMPARATIVE ANATOMY. 



become differentiated into incisors, canines, and grinders 

 (premolars and molars). 



Fishes and Amphibia. The dermal denticles already de- 

 scribed in the chapter on the skeleton are structures homologous 

 with teeth, for both are developed in a similar manner. In 

 Teleostei 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 

 same 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 



Fio. 174. SKULL OF Batrachoseps attcnuatus. (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 larvae of Salamanders and 

 in Proteus the splenial of the lower jaw is also toothed. Horny 

 jaws and horny teeth are present in larval Anura. 2 



1 Tn Fishes the teeth may be cylindrical, conical, or hooked ; or in some cases 

 (Scarus, and the Sarginas) they may be chisel-shaped, resembling the incisors of 

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

 pavement, are rounded in form, and serve to crush the food. Again, they may be 

 delicate and bristle-shaped (Chaetodou), or sabre-shaped (Chauliodus). 



2 The horny structures .on the upper jaw in young stages of Polypterus, in the 

 mouth of Cyclostonji, on the jaws of Siren lacertina, and on the lips of Dipnoi, belong 

 to the same category as these. 



