ENAMEL 105 



tion with the dentine there is no appearance of the spaces 

 into which the tubes pass, so conspicuous in the enamel of 

 Macropus. The tubes bend slightly and often branch at 

 their entry into the enamel, but otherwise pass uninter- 

 ruptedly across the boundary. In other respects the enamel 

 is very similar to that of marsupials, but the absence of 

 the dilatations points probably to a more complete calcifica- 

 tion of the interprismatic material in Hyrax, a point which 

 will be further considered in treating of the development 

 and calcification of the enamel in marsupials. 



As previously mentioned, penetration of the enamel by 

 tubes continuous with those from the dentine is also seen 

 in the shrews (Sorex), among the Insectivora, and in the 

 Rodents in Jerboa. Slight penetration of the enamel from 

 the dentine is also seen in many Garni vora (18/), and has 

 been described by the author in the molar tooth of the 

 Elephant (1 1 a), and may often be seen in human teeth where 

 occasionally scattered tubes penetrate to a considerable depth. 



These examples of tubular enamel, as Tomes says, ' dis- 

 tinctly point to this penetration of the epiblastic enamel 

 by tubes continuous with those of the mesoblastic dentine, 

 being a primitive character, to which some slight tendency 

 to revert has not been quite lost by placental mammals '. 



Owing to some resemblances to the dentition of mar- Oeodonts. 

 supials found in the teeth of Creodonts, C. S. Tomes was 

 led to examine numerous teeth of this fossil sub-order (the 

 members of which are considered by most authorities to be 

 the ancestors of the Carnivora, and possibly also of the 

 Insectivora), and to determine if the histological characters 

 of the teeth showed any resemblance to those of marsupials. 

 The conclusions he arrived at from the examination of teeth 

 of Hyenodon, Mesonyx, and many other forms, were that, 

 so far as the structure of the enamel was concerned, none 

 of these animals show any greater resemblance to mar- 

 supials than do the recent Carnivora, but the patterns of 

 the enamel in Creodonts closely resemble those of the 

 Carnivora. Tubular enamel was not found in the teeth of 

 the Creodonts examined, and so far as the histological 

 evidence is concerned, they do not show any affinity to 

 marsupials (18/). 



