ENAMEL 



85 



he places it under the heading of ' Enamels which are not 

 wholly epiblastic ', 'a tissue which is laid down by the 

 operation of epiblastic ameloblasts in a matrix which is 

 derived from a modification of the surface of the meso- 

 blastic dentine papilla. Rose is, however, inclined to look 

 upon these tissues as dentine, but on the whole of the 

 evidence afforded by the structure and development of 

 this tissue in Elasmobranchs, Tomes considers it may be 

 appropriately called ' enamel '. 



en 



FIG. 40. Enamel and osteodentine of Heterodontus (Ostracion). 

 Fuchsin stain by capillary attraction. ( X 50.) 



In Cestracion Philippi (Heterodontus), the Port Jackson Hetero- 

 shark, tubes are seen to penetrate the outer layer from (c es tra- 

 without, as well as from the dentine. This fish is of especial CMm )- 

 interest, as the family to which it belongs, represented by 

 four species, are the sole living representatives of a family 

 of fishes which were the most characteristic and abundant 

 sharks of the Mesozoic Period. 



Their rounded teeth are eminently adapted for crushing 

 the hard-shelled animals which form their food. The surface 

 of these teeth is marked by rows of little pits or depressions, 

 and a longitudinal section shows radiating lines passing 

 from the bottom of these pits into the enamel. When 



