224 LECTURE IX. 



porting plate gradually undermined by the growth of the new tooth 

 must have given way and been forced upon the subjacent delicate 

 and highly vascular and sensitive matrix of the half-formed tooth. 

 But the superincumbent pressure being exclusively sustained by the 

 border of the alveolus, whence it is transferred to the walls dividing 

 the vaulted cavities containing the germs of the new teeth, the roofs 

 of these cavities yield to the absorbent process consequent on the 

 growth of the new teeth without materially weakening the attach- 

 ment of the old teeth, and without the new teeth being subjected to 

 any pressure until their growth is sufficiently advanced to enable 

 them to bear it with safety ; by this time the sustaining borders of the 

 old alveolus are undermined, and the old worn-down tooth is shed. 



With regard to the substance of the teeth of fishes, the modifica- 

 tions of dentine, called vaso-dentine, and osteo-dentine*, predominate 

 much more than in the higher Vertebrata ; and they thus more closely 

 resemble the bones which support them. There is, however, great di- 

 versity in respect of substance. The teeth of most of the Chcetodonts 

 are flexible, elastic, and composed of a yellowish subtransparent albu- 

 minous tissue ; such, likewise, are the labial teeth of the Helostome, the 

 premaxillary and mandibular teeth of the Goniodonts, and of that 

 percoid genus thence called Trichodon. In the Cyclostomes the teeth 

 consist of a denser albuminous substance. The upper pharyngeal 

 molar of the Carp consists of a peculiar brown and semitransparent 

 tissue, hardened by salts of lime and magnesia. The teeth of the 

 Flying-fish (^ExoccEtus), and Sucking-fish (^Hemora), consist of osteo- 

 dentine. In many fishes, e.g. the Acanthurus (V, pi. 44. fig. 1.), 

 Sphyrjena (V, pi. 53.), and certain Sharks {Lamna, V, pi. 6.), abase, or 

 body of osteodentine is coated by a layer of true dentine, but of un- 

 usual hardness, like enamel: in Frionodoni\\i^\\^vdi tissue predominates. 

 In the Diodon the dental plates consist wholly of hard or unvascular 

 dentine. In Sargus and Balistes the body of the tooth consists of 

 true dentine, and the crown is covered by a thick layer of a denser 

 tissue, developed by a distinct organ, and difiering from the ' enamel' 

 of higher animals only in the more complicated and organised mode 

 of deposition of the earthy salts. The ossification of the capsule of 

 the complex matrix of these teeth covers the enamel with a thin 

 coating of ' cement.' In the pharyngeal teeth of the Scams a fourth 

 substance is added by the ossification of the base of the pulp after its 

 summit and periphery have been converted into hard dentine ; and 

 the teeth, thus composed of cement, enamel, dentine, and osteodentine 

 (V, pi. 52.), are the most complex in regard to their substance that 

 have yet been discovered in the animal kingdom. 



* V. Introduction, p. Ixxii. 



