PISCES. 



979 



Fig. 514. 



Teeth of the Tench. 



a, roof of the mouth ; b, the oesophagus ; c, dental 

 projection from basilarbone j d, d, pharyugeal teeth. 

 (After Owen.) 



514), and thus forms a kind of anvil upon 

 which the bruising pharyngeal teeth play, and 

 thus crush and triturate whatever food passes 

 into the oesophagus. 



In the Scari, which have to feed upon the 

 numerous corallines that clothe the rocks at 

 the bottom of the ocean, the dental apparatus 

 given to protect their jaws from injury while 

 biting such hard substances is very remarkable. 

 These Fishes have their jaws, which resemble 

 the beak of a parrot (whence they receive their 

 usual appellation " Parrot-fishes,") covered 

 externally with a kind of pavement of teeth 

 answering the same purpose as the horny 

 investment of the mandibles of the bird. The 

 teeth that form this pavement are perpetually 

 in progress of developement towards the base 

 of the jaw, whence they advance forwards, 

 when completed, to replace those which become 



Fig. 516. 



In the Cyprinidtf, or Carp- ge- 

 nus, the bones composing the 

 superior and inferior maxillae are 

 completely edentulous, but to 

 make up for this deficiency the 

 pharyngeal bones are armed with 

 a remarkably powerful dental ap- 

 paratus of very singular character. 

 Each of the inferior pharyngeal 

 bones, which are exceedingly 

 strong and form a kind of osseous 

 framework at the commencement 

 of the oesophagus, supports four or 

 five large teeth of great strength. 

 These are opposed to a single 

 dental piece of stony hardness and Section of the jaw of the Parrot-fish, shewing the progress of dentition. 



laminated structure, which is fixed 



c, teeth still enclosed in the jaw ; I, do. with their extremities 



... - . . _ - \ff v^v,LUi Ob&AI ^li^lVFO^Vi 111 LrUtJ 1 w . ' , *' 



upon a dilated projection from the protruded so as to form an external pavement. (After Owen.) 



basilar bone of the cranium (Jig. 



worn away in front by the constant attrition to 

 ig. 515. which they are subjected. 



In the annexed figure (Jig. 515) the ex- 

 ternal appearance of these singularly disposed 

 teeth is well represented, while in the vertical 

 section (fig. 516) the mode of their implan- 

 tation into the anterior surface of the jaw is 

 delineated. All these teeth were originally 

 developed in a common alveolar cavity (516, c) 

 situated in the substance of the jaw. The 

 outer* wall of this cavity is much weaker 

 than the dense and compact inner wall, and 

 moreover it becomes thinner as it approaches 

 the margin of the jaws and disappears (fig. 

 516, /,) at different distances in different spe- 

 cies of Scari before it reaches that margin. 

 Where it exists at the base of the jaws it is 

 sometimes, as in Scarus muricatus represented 

 in the figure, perforated by numerous small 

 foramina, through which foramina in the recent 

 fish processes of the external periosteum are 

 continued to the analogous membrane lining 

 the dentigerous cavity and forming the capsule 

 of each denticle. These processes are analo- 

 gous to the gubernacula of the second series of 

 teeth in the Mammalia, and like them serve 

 to conduct the new teeth to the exterior of the 

 jaw. The growing denticles (516, /,) be- 



* Owen, Odontography, page 115. 

 3 R 2 



Beak of Parrot-fish (Scarus muricatm). 



a, upper mandible, the exterior of which is com- 

 pletely encased in teeth ; 6, lower mandible par- 

 tially so, the inferior teeth not having as yet pro- 

 truded ; c, lateral fangs. (After Owen.) 



