THE SKELETON IN FISHES. THE TEETH. Ill 



ankylosed to the bone that bears them. Such teeth are, 

 with a few exceptions such as Balistes, not imbedded in 

 sockets nor replaced vertically. 



In some fish beak-like structures occur, formed partly of 

 teeth, partly of the underlying jaw bones. These beaks are of 

 two kinds : (1) In Scarus, the parrot fish, the premaxillae and 

 dentaries bear numerous small, separately developed teeth, 

 which are closely packed together and attached by their 

 proximal ends to the bone, while their distal ends form a 

 mosaic. Not only the teeth but the jaws which bear them 

 are gradually worn away at the margins, while both grow 

 continuously along their attached edge. (2) In Gymnodonts, 

 e.g. Diodon, the beaks are formed by the coalescence of broad 

 calcified horizontal plates, which when young are free and 

 separated from one another by a considerable interval. 



In some Teleosteans the differentiation of the teeth into 

 biting teeth and crushing teeth is as complete as in Lepi- 

 dosteus. Thus in the Wrasse (Labrus), the jaws bear conical 

 slightly recurved teeth arranged in one or two rows, with some 

 of the anterior ones much larger than the rest. The bones of 

 the palate are toothless, while both upper and lower pharyngeal 

 bones are paved with knob-like crushing teeth ; such pharyn- 

 geal teeth occur also in the Carp but are attached only to the 

 lower pharyngeal bone, the jaw bones proper being toothless. 



In DIPNOI the arrangement of the teeth is very similar to 

 that in Holocephali. The mandible bears a single pair of 

 grinding teeth attached to the splenials, and a corresponding 

 pair occur on the palato-pterygoids. In front of these there 

 are a pair of small conical vornerine teeth loosely attached to the 

 ethmoid cartilage. The palato-pterygoid teeth of Ceratodus 

 are roughly semicircular in shape with a smooth convex inner 

 border, and an outer border bearing a number of strongly 

 marked ridges. The teeth of the extinct Dipteridae resemble 

 those of Ceratodus but are more complicated. 



