124 



THE STEUCTUKE OF THE HOESE 



lower jaws, it will be necessary to describe them sepa- 

 rately. 



Of the six principal teeth which constitute the upper 

 molar series, the four middle ones (the last two pre- 

 molars, pm 3 and _pm 4 , fig. 16 ; and the first two molars, 

 m 1 and m 2 ) are almost exactly alike in size, form, and 

 structure, being, roughly speaking, four-sided prisms 

 with a nearly square section. The foremost, _pm 2 , and 

 the hindermost, m 3 , differ from the others, being more 

 triangular in section, the apex of the triangle pointing 

 forwards in the first and backwards in the last. 



To understand properly the arrangement of the 

 enamel folds and of the dentine within, and the cement 

 on the outside of them, as seen in a section or in the 

 naturally worn surface of one of the middle teeth, 

 it is necessary to examine it in its unworn and even 

 unfinished state, before the thick coat of cement has 

 been deposited around the ridges and projections of the 

 surface. Such teeth can always be found within the 

 sockets of the prepared skulls of young animals, and 

 deserve careful study, on account of the light they throw 

 upon the structure of the organ in its maturity. It will 

 be seen (fig. 21) that the pattern presented by the free 

 surface is essentially similar to that of the corresponding 

 tooth of Anchitherium, which is itself a modification of 

 that of Hyracotherium, from which it is but an easy 

 transition to Phenacodus, as shown in the first chapter. 



