144 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



required they are not always confined to the bones 

 of the skull ; thus, in the just-mentioned Scarus, the 

 lower pharyngeal bones unite, and they, like the 

 upper pharyngeals, are armed with crushing teeth (Fig. 

 63) ; here, then, we have an instance of the bones of the 

 branchial arches (see page 328), being tooth-bearing. 

 Another example is afforded by the carp, in which fish 

 the bones of the skull are all devoid of teeth, which 

 are confined to the lower pharyngeals ; these, as in 

 the case of the incisors of the sheep or ox, do not 

 work on upper teeth, but on a hard process, which, 

 in the carp, is developed on the occipital bone of the 

 skull. 



In other fishes the teeth are exceedingly deli- 

 cate, as in Chcetodon, which has gained its name 

 from the bristle-like character of these organs. In 

 a few cases the teeth are placed in distinct sockets, 

 as in the file-fishes, of which Balistes is an example ; 

 in Lepidosteus the socket is not complete, and the 

 tooth becomes anchylosed to its walls. 



Lepidosiren presents an arrangement not unlike 

 that which is found in Rodents among Mammals, for 

 the front edge of the teeth is harder than the rest, 

 which therefore wears down sooner, and leaves a sharp 

 cutting edge. In no group of the Vertebrata are these 

 organs of greater value to the palaeontologist than 

 among Fishes, as the discovery in Australian rivers 

 of Ceratodus, which had been thought to have been 

 extinct since the time of the deposit of the older 

 secondary rocks, is sufficient to bear witness. 



In a few cases there are differences between the 

 teeth of males and females, as in the skate (Fig. 

 64 ; A and B) where those of the male are more 

 pointed than those of the female. 



In the male salmon, at the breeding season, the 

 lower jaw is produced into stout hooks, and in corre- 

 spondence with this the anterior end of the upper jaw 



