TEETH 



121 



folded dentine developed round a pulp with outstanding ridges 

 (Lepidosteus, Fig. 81, B) ; vasodentine, with branching pulp- 

 channels, but few or no canaliculi (Merlucius, Fig. 81, A); osteo- 

 dentine, or trabecular dentine, with anastomosing branches of 

 the pulp-cavity (Esox, Fig. 81, C; Lamna) ; vitrodentine is 

 the name given to the outermost enamel-like layer generally 

 found on Elasmobranch teeth and denticles, as described above. 

 Intermediate forms exist between these varieties, and occasionally 



A. 



FIG. 81. 



A, section through a tooth and jaw of the Hake, Merlutius vulgnris. (Partly after Tomes.) 

 B, transverse section of the tooth of Lepidosteus. C, enlarged view of a small piece of the tooth 

 of Esox Indus. 1, enamel tip; 2, vasodentine; 3, elastic ligament; 4, bone of jaw; 5, free 

 outer edge of tooth-base ; 0, bone of attachment forming socket for tooth-base ; 7, pulp-cavity ; 

 8, dentine layer at surface of tooth ; 9, osteodentine canals ; 10, folded wall of dentine. 



dentine may come to resemble bone$ with laminae and enclosed 

 cells. 



The placoid scales do not indefinitely increase in size. New 

 denticles may develop from fresh * germs ' between the old ones, 

 and these when old and worn out may be shed (Steenstrup). 



Along the inner margin of the jaws, the teeth, which are merely 

 specialised denticles, arise at the bottom of a deep dental groove 

 (Fig. 39). Here new generations of teeth are continually being 

 produced, and as they grow older they move up to the biting edge 

 of the jaw, pushing the older teeth before them. Thus the old 



