126 MR. 0. THOMAS ON CRANIAL VARIATION. __[Feb. 16, 
later life among this group of animals, that I have thought it worth 
while to have them figured, and to make a few notes on the causes 
and extent of the changes presented by them. This is the more 
called for owing to Mammalogists who have not access to large 
series of specimens being as a rule far too ignorant of these changes 
to judge by the numerous so-called “new species’? which are 
constantly being described on variations of a corresponding nature. 
These changes have been pointed out at various times, as for example 
by Berthold’, Nathusius*, Hensel *, and others; and the object of 
the present paper is therefore merely to give further publicity to a 
fact which has a very considerable bearing on the value of the cranial 
differences brought forward as characteristics of freshly-described 
species. 
The three skulls, which I will speak of as A, B, and C, consist 
evidently of two males and one female, the latter being far smaller 
and lighter than the other two. Of their specific identity there can 
be absolutely no question, although I have found it difficult to make 
most people to whom I have shown them believe in that fact. 
B (Plate XT. fig. B) is a skull of the ordinary type, showing 
no special peculiarities ; it is fully adult, as is proved by the basilar, 
sphenoid, and other cranial sutures being closed, although some of 
the facial ones are still visible, and by the teeth showing signs of 
commencing wear. Compared to such a skull as this, the aged skull 
A (fig. A) shows the changes that take place in later life to a most 
remarkable extent, well worthy of some special notice. 
All the changes are connected with an increase in the power of 
biting, and the consequent development of the biting muscles, but 
the connection of some of the changes with the biting muscles is not 
so obvious at first sight as to be readily perceived by any one not 
specially on the look-out for it. 
To commence with, the canines of A are apparently very much 
longer and more powerful than those of B, a difference not due to 
individual variation, but to the fact that they are as a whole pushed 
further outwards as time goes on and additional dentiné is deposited 
round their bases ; the absolute length of the enamel-covered portion 
from the cingulum to the tip being precisely the same in both. 
The next difference is one too commonly recognized to need much 
remark, viz. the development of the occipital crest; but it is here 
carried to a relative extent greater than in any other skull I know 
of, not even excepting the Hyzenas and Sea-lions. In B the crest 
has scarcely commenced to grow at all, while in A it is no less than 
20 millim. (nearly an inch) in vertical height above the brain-case. 
The temporal muscles, passing from these enormous crests down 
through the zygomata to the lower jaw, have then by their constant 
pressure on the bone caused it to absorb in certain places, and have 
thus induced three remarkable changes in the skull that might easily 
be, and have in fact often been, taken as marks of specific distinction. 
Firstly, they have pressed against and bowed outwards the zygomata, 
1 Tsis, 1830, p. 570. 2 Arch, f. Nat. iv. p. 130 (1838). 
3 “ Craniologische Studien,” N. Act. Ac. Nat. Cur. xlii. p. 127 (1881). 
