PROBOSCIDEA 421 



corresponding tooth of one of the primitive Artiodactyles into that 

 of an Ox. The intermediate stages, moreover, even in the present 

 state of our knowledge, are so numerous that it is not possible to 

 draw a definite line between the two types of tooth structure (see 

 Fig. 179, I, II, III, IV). 



As regards the mode of succession, that of modern Elephants is, 

 as before mentioned, very peculiar. During the complete lifetime 

 of the animal there are but six molar teeth on either side of each 

 jaw, with occasionally a rudimentary one in front, completing the 

 typical number of seven. The last three represent the true molars 

 of ordinary mammals; those in front appear to be milk -molars, 

 which are never replaced by permanent successors, but the whole series 

 gradually moves forwards in the jaw, and the teeth become worn 

 away and their remnants cast out in front, while development of 

 others proceeds behind. The individual teeth are so large, and the 

 processes of growth and destruction by wear take place so slowly, 

 that not more than one, or portions of two, teeth are ever in place 

 and in use on either side of each jaw at one time, and the whole series 

 of changes coincides with the usual duration of the animal's life. 

 On the other hand, the Dinotherium, the opposite extreme of the 

 Proboscidean series, has the whole of the molar teeth in place and 

 use at one time, and the milk-molars are vertically displaced by 

 premolars in the ordinary fashion. Among Mastodons transitional 

 forms occur in the mode of succession as well as in structure, many 

 species showing a vertical displacement of one or more of the milk- 

 molars, and the same has been observed in one extinct species of 

 Elephant (E. planifrons) as regards the posterior of these teeth. 



All known Proboscideans are animals of comparatively large 

 dimensions, and some are the most colossal of land mammals. The 

 head is of great proportionate size ; and, as the brain case increases 

 but little in bulk during growth, while the exterior wall of the 

 skull is required to be of great superficial extent to support the 

 trunk and the huge and ponderous tusks, and to afford space for 

 the attachment of muscles of sufficient size and strength to wield 

 the skull thus heavily weighted, an extraordinary development of 

 air-cells takes place in the cancellous tissue of nearly all the bones 

 of the cranium (Fig. 180). These cells are not only formed in the 

 walls of the cranium proper, but are also largely developed in the 

 nasal bones and upper part of the premaxillae and maxillae, the bones 

 forming the palate and the basicranial axis, and even extend into 

 the interior of the ossified mesethmoid and vomer. Where two 

 originally distinct bones come into contact, the cells pass freely 

 from one to the other, and almost all the sutures become obliterated 

 in old animals. The intercellular lamellae in the great mass which 

 surrounds the brain cavity superiorly and laterally mostly radiate 

 from the inner to the outer table, but in the other bones their 



