382 



NATURAL SCIENCE. 



June, 



primitive independence and are not directly connected with the jaws' ; 

 they are simply embedded in the tough fibrous gum (Fig. 9, m.), 



Some teleostean fishes are provided with teeth which yield to 

 pressure in a certain direction, and subsequently resume the upright 

 position, by the elasticity of the ligament which attaches them to the 

 jaw. In the angler (Lophius), in which these teeth acquire consider- 

 able size, the front of the base of each tooth is free, while, on the 

 posterior side, dense fibrous ligaments radiate from the tooth to the 

 bone of the jaw. The teeth, therefore, bend readily inwards towards 

 the back of the mouth, but cannot be made to point outwards without 



Figs, i and 2.— Vertical sections, greatly enlarged, of teeth oi Cestracion (i) and 

 Lamna (2) (after Tomes), d, hard, unvascular dentine; vd, vasodentine ; 0^, osteo- 

 dentine ; e, enamel. Fig. 3.— Upper and lower teeth of Notidanns gnseiis. Fig. 4.— 

 Upper and lower teeth of Carcharias lamia. Fig. 5.— Tooth of Odontaspis eUgans, 

 Mid. Eocene. Fig. 6.— Tooth of Cladodtis stviaUis, Lr. Carboniferous. Fig. "7.— 

 Portion of lower jaw of skate, Raia batis, male, seen from behind. Fig. 8.— Portion 

 of lower jaw of thornback, Raia clavata, female, from behind. 



breaking the ligament. In the hinged teeth of the pike {Esox) the 

 elasticity resides, not in the hinge proper, but in elastic fibres running 

 from the osteodentine, that occupies the interior of the tooth, to the 

 bone of the jaw. 



1 No true ossification takes place in the jaws of Elasmobranch fishes. A layer of 

 calcified tesserae is frequently found on the surface of the cartilage in the form of 

 a crust, and in some e.xtinct forms, e.g., Pleuracanthus and Cladodus, as also in the 

 recent Actobatis, Myliobatis, and RhinopUra, the cartilage is permeated throughout by 

 granular calcifications. 



