126 MR. O. 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, sliowing 

 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 diflference not due to 

 individual variation, but to the fact that they are as a whole pushed 

 further outwards as time goes on and additional dentine 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 or 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 Hyaenas 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, 

 ^ Isis, 1830, p. .570. 2 ^rg]j f jjat. iv. p. 1.30 (1838). 



^ " Craniologische ibtudien," N. Act. Ac. Nat. Our. xlii. p. 127 (1881). 



