PTYCHODUS. 



227 



The microscopic structure of the teeth has already been described by Agassi/' 

 and Owen. 2 They always consist of vaso-dentine, with a moderately thick layer 

 of gano-dentine on the coronal surface ; and they have no internal cavity (Text- 

 fig. 72). The vaso-dentine of the root is of very open texture, with the medullary 

 canals irregularly verrniculating ; while that of the crown is traversed by canals 

 which rise towards the surface, diverging from each other and branching 

 dichotomously, so as to maintain a direction vertical to the surface towards which 

 they proceed. From each medullary canal minute calcigerous tubules radiate 



Fig. 71. Ptychodus decurrens, Agassiz ; diagram of mandible, oral aspect, showing arrangement of teeth on 

 symphysis, based on specimen shown in Text-fig. 70. From Quart. Journ. Geol. Soc, vol. lx (1904). 



outwards, those near the base at right-angles, those nearer the surface at more and 

 more acute angles, until the canal itself terminates just below the translucent gano- 

 dentine in a tuft of such tubules. The calcigerous tubules are more wavy than 

 the medullary canals, rapidly branch and subdivide to extreme minuteness, and 

 finally terminate by anastomosing with each other. A transverse section (Text- 

 fig. 72 B) shows that the medullary canals do not form the centres of well- 

 separated prismatic denticles, such as are always distinct in the teeth of the 

 existing Myliobatis, Bhinoptera, and lEtobatix. 



Teeth occasionally occur in which the enamel-layer has been destroyed either 



1 L. Agassiz, Poiss. Foss., vol. iii (1843), p. 162, pi. K, figs. 1, 2. 



2 R. Owen, Odontography (1840), p. 57, pis. xviii, xix. 



