THE EXTERNAL SKELETON. 



A similar but less extreme complexity of structure cha- 

 racterizes the last molar of the largest 'animal of the Rat 

 order, namely, the so-called River-hog of the La Plata 

 Hydrochcerus. 



Foldings of the dentine, bounded by enamel and with 

 cement filling the valleys and interspaces, produce such com- 

 plex patterns as we find in the Horse and Sheep already 

 described, and also in the Porcupine and Beaver. 



Molar teeth may grow (like those of man) from roots which 

 soon become calcified, so that no more growth can take place ; 

 and this is the more common condition of the Mammalian 

 molars. 



They may have very long roots capable of a prolonged but 

 still limited increase, as in the Horse. 



They may spring from persistent roots growing during the 

 whole of life, as in the Porcupine. 



The pulp-cavity may be curiously disposed or divided. 

 Thus in poisonous Serpents it extends round the greater part 

 of the poison canal, which, it need hardly be said, is really 

 outside the tooth. It may be divided transversely, as in the 

 incisor of the Horse (Fig. 230), where one branch ascends in 

 front of the depression (or " mark "), and the other branch 

 ascends behind it. It may 

 be divided antero- pos- 

 teriorly, as in notched in- 

 cisors, and especially in the 

 comb-like ones of the Fly- 

 ing Lemur, where (Fig. 

 228) a branch of the pulp- 

 cavity ascends each pro- 

 cess of the "comb." 



But the most remark- 

 able form of the pulp- 

 cavity is that which ex- 

 isted in those extinct Ba- 

 trachians the Labyrintho- 

 dons. In those creatures, 

 numerous vertical grooves 

 on the surface penetrate 

 with many inflections 

 deeply into the substance 



of the teeth, and similar narrow processes of the pulp- 

 cavity interdigitate, also with many inflections, between the 

 inwardly extending grooves. With this structure a transverse 

 T 2 



253. ONE QTARTER OF A MUCH 

 ENLARGED HORIZONTAL SECTION OF 

 THE TOOTH OF A LABYRINTHODON. 



