1x DENTITION 249 
dermal spines form a continuous series with those which invest 
the jaws and eventually become teeth (Fig. 151). It is only 
later, when lips become apparent, that the continuity of the 
teeth and dermal spines is interrupted, and the two structures 
assume their distinc- 
tive characters. 
The tissues of which 
the teeth of Fishes are 
composed are (1) den- 
tine, which is a non- 
vascular, calcified 
tissue, traversed by 
numerous radiating, 
branched, dentinal 
tubuli, into which ex- 
tend protoplasmic pro- 
longations from the 
cells (scleroblasts) by 
which the dentine is 
secreted. Dentine 
forms the greater part 
of the body of a tooth. 
(2) vasodentine and 
(5) osteodentine are 
modifications of ordi- 
nary dentine the Fic. 151.—Transverse section through the lower jaw of 
ce ae an embryo Scyl/ium, to show the gradual transition 
former containing from dermal spines (d, d, d@) on the outer surface of 
blood - vessels ramify- the jaw to teeth (¢, ¢, ¢) on the oral surface. c, Car- 
) tilage of the lower jaw. (From Gegenbaur.) 
ing in its substance 
but. no dentinal tubules, and the latter more closely resembling 
bone. (4) enamel, an exceptionally dense, non-vascular, non- 
tubular tissue, which may or may not exhibit traces of the 
prismatic structure so characteristic of this tissue in the higher 
Vertebrates, forms the outer investment of the teeth. 
As regards their fixation, the more primitive forms of teeth, 
such as those of Elasmobranchs, are simply embedded in the 
gums, and are only connected with the jaws by fibrous tissue ; 
but in some: of the older fossil Sharks the fixation of the teeth 
is effected by the mutual articulation of the basal plates of the 
teeth with one another. The Chondrostean Polyodon, so shark- 
