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, Il, 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 cither 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 (£4. plunifrons) 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 (Fie. 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 premaxilla and maxilliv, 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 lamelle 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 
