46 MR. C. S. TOMES ON THE [Jan. 16, 



matter little and would remain constant. But it was shown by 

 my father, the late Sir John Tomes (5), that by a mere examina- 

 tion of sections of the enamel it was possible in the case of 

 Rodents not merely to pronounce that the enamel was that of a 

 Rodent, but in a large number of instances to refer it correctly to 

 a particular family of Rodents, or to a group of families. 



In the moi-e simple forms of enamel, the enamel prisms all pass 

 outwards from the surface of the dentine to the outer surface of 

 the enamel, and are, with very slight exception, exactly parallel 

 with one another. 



Bu.t in the Rodents contiguous layers of enamel prisms start off 

 from the dentine at different angles, the layers alternating in this 

 respect, so that if the section embraces in its thickness more than 

 one layer, as such sections almost invariably do, patterns are 

 prodviced by the crossing of the prisms, and these patterns are 

 constant and characteristic of many of the families. 



Similarly, my father showed that the enamel of Marsupials (6) 

 presented characters very unusual in Placental mammals, and 

 therefore almost characteristic of Marsupials, whilst the Carnivora 

 also presented well-marked enamel characteristics. 



It therefore seemed to be well worth while, in view of the 

 uncertainty of the position of the Creodonts and of their relation 

 to recent Carnivora, and possibly to the Marsupials, to apply this 

 test of their affinities. 



As bearing upon the subject generally, I may mention that I 

 myself examined the teeth of a number of genei'a of the Gadidee 

 (7), a family selected as being both fairly numerous and at the 

 same time compact, with the resvilt that I found that the enamel 

 was alike in all, but that the dentine presented mai-ked variations 

 upon a common type of vasodentine, and that these peculiarities 

 coincided with their zoological arrangement, and not with the 

 functional development of the dentition. Thus some of the 

 largest teeth presented the simplest, and almost degraded, struc- 

 ture common to them and their immediate relations, whilst some 

 teeth, reduced so as to be almost rudimentary, retained the com- 

 plexity of sti'ucture characteristic of their zoological relations. 

 These, however, are the only papers I have met with in which 

 this line of research had been followed out to any extent. 



In what may be termed the norm.al arrangement of the dental 

 tissues of placentpJ mammals, the tubes of the dentine end by 

 branching and becoming very fine, or by entering minute globular 

 or angular spaces within the boundaries of the dentine (see 

 text-figs. 15, 16, & 19, pp. 50, 51, 53) ; but it was shown by my 

 father that in Marsupials the greater number of the dentinal 

 tubes, instead of so ending, became continuous with tubes 

 which traverse the enamel. This is true of all Marsupials, 

 recent or extinct, which were examined by him or by myself 

 at later dates, with the solitary exception of the "Wombat, in 

 which this does not happen, though, as might be expected from 

 what has already been said, the precise extent to which it hapjoens 



