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Show 126 MR. O. THOMAS ON CRANIAL VARIATION. [Feb. 16, later life among this group of animals, that 1 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 2, Hensel3, 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 w h o m I have shown them believe in that fact. B (Plate X L 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 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 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 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, 1 Isis, 1830, p. 570. 2 Arch. f. Nat. iv. p. 130 (1838). 3 " Craniologische Studien," N. Act. Ac. Nat. Cur. xlh. p. 127 (1881). |