254 MICROSCOPIC ANATOMY OF THE TEETH 



dentine in which the tubes are given off from the foldings 

 of the pulp, as Tomes says, ' like a paddle-wheel '. 



These radiating portions are sometimes branched and 

 subdivided, showing a still more complicated pattern, and 

 the effect of a convoluted tubular system is given as in the 

 fossil Labyrinthodon (fig. 56). 



As shown by Tomes there are two systems on which 

 plicidentine is formed : one in which the foldings may be 

 described as lateral, as in the lizard Varanus, in the 

 bony pike, Lepidosteus, and in Labyrinthodon, and another 



FIG. ] 56. Transverse section of tooth of Labyrinthodon showing highly 

 convoluted plicidentine. Reduced from Owen's Odmitography. 



in which parallel vertical* columns of the pulp give rise to 

 radiating dentinal systems around them, these systems 

 lying side by side and the terminations of the tubes of one 

 system often communicating with those from neighbouring 

 columns (figs. 157 and 158). 



This latter structure is seen in the Rays, Myliobates, and 

 Zygobates (fig. 155), andin the rostrum of the saw-fish (Pristis) 

 and in the teeth of the Cape Ant-eater (Orycteropus) . In the 

 lower part of the tooth in these animals the pulp is more 

 or less fused by the union of the pulps of the separate 

 denticles. ' Instead of being regarded as a plicidentine 

 such a tooth might be said to be built up of a series of 

 small, parallel, fused denticles, or exceedingly broadened 

 and fused cusps.' * 



1 C. S. Tomes, Dental Anatomy, 7th ed., p. 89. 



