GENEKAL STRUCTURE OF THE TEETH. 465 



ing process, except that no true bone is formed, but only an 

 allied substance of much harder consistence, and differing more 

 or less in histological structure, termed dentine. The epithelium 

 of the tooth papilla either atrophies to a rudimentary horny 

 investment, the cuticula (membrane of the enamel), or it 

 becomes elongated in a remarkable manner into long petrified 

 prisms, which collectively invest the dentine, and are known as 

 the enamel. In addition to these there is found an accessory 

 structure, the cement, a true bony substance, which especially 

 invests the fangs of the teeth. Dentinal teeth are con- 

 stantly attached to the parts of the skeleton surrounding the 

 mouth and pharynx, and for the most part to the lower jaw. 



From the simple arrangement of the three chief constituents of 

 the teeth, as they occur in man, for example, there are manifold and 

 complex variations. Amongst these may be enumerated in particular 

 the so-called folded enamel teeth of Bodentia, Solipedes, and others, and 

 the compound teeth of many fishes and fossil reptiles (Labyrinthodon), 

 of the elephant, etc. The "folded enamel" teeth, dentes complicati, 

 are formed on the type of a simple tooth. The dentine of the crown 

 is, however, folded like a ruff, and the enamel and cement dip in to 

 form a covering to the surface of all the sinuosities. Of the dentes 

 compositi two principal forms can be distinguished. In one, a common 

 stem or trunk is present, which gives off a number of separate tooth- 

 lets (G-aleopithecus, Labyrinthodon), whilst in the second a common 

 tooth pulp is absent, and instead we find, as in many fishes and 

 Orycteropus, numerous independent toothlets proceeding from the jaw, 

 and united to form a common tooth. The pulp of the teeth of the 

 Labyrinthodonts is therefore comparable to the compound filiform 

 papillae of the tongue ; whilst the true compound teeth of the second 

 class bear the same relation to simple teeth that the hoof does to hair. 

 The several back teeth of the Elephant have the characters of the 

 first kind; each separate tooth, however, presents folding of the 

 enamel, so that a highly complex structure results. 



On the other hand, the structure of a tooth may be simplified by 

 the absence of one or two of the above-mentioned dentinal tissues, 

 especially the enamel, or the enamel and cement. Thus the tusks of 

 the Elephant and the teeth of the Edentata have no enamel ; and 

 again, in the case of the Rodents, the masticating surface of their 

 incisor teeth has no enamel. According to Owen (34), the pharyngeal 

 teeth of Labrus are composed of ordinary dentine alone. Amongst 



