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Show 1885.] MR. O. THOVIAS ON THE RACES OF ECHIDNA. 333 these being, in the absence of teeth, the only characters available comparison. With regard to age the following are the measurements of two skulls, both of var. aculeata, and both from Central Queensland : - i'. Adult. /. Young i\ Adult. f. Young Taking these two as examples, we see that young skulls have comparatively large rounded brain-cases, short snouts, and broad interorbital spaces. In growing older the size of the brain is nearly unaffected, but the rostrum lengthens and seems to become more distinctly bent upwards ; the sutures close, and the various fonta-nelles fill up, with the exception that the vacuities on the base of the skull, just in front of the condyles (the "condyloid vacuities"), when present, do not apparently close until extreme old age. Altogether, however, there is probably more difficulty in determining the age of specimens of this group than in any other mammals, chiefly of course owing to the want of teeth; and it is only by a comparison of a considerable series that any satisfactory estimate of age can be made. With regard to the condyloid vacuities another element than age seems to enter into the question. Some specimens, although quite young, have no vacuities (e.g. specimen/), while others fully adult, such as r, s . u', &c, have large and open ones (see Plate XXIV. figs. E and F) ; and this seems to depend in a large measure on locality, as very nearly all the specimens of E. setosa that I have seen have open vacuities, while without exception the long-spined northern examples have closed ones. This cannot, however, be used as a specific character, as is shown by the fact that the skull z', from Tasman's Peninsula, Tasmania (No. 3957, Coll. Surg.), although only half- 1 From the tip of the premaxillse to the most posterior point of either condyle. 2 Length : breadth : : 100 :- 3 From the centre of the lower edge of the foramen magnum (basion of anthropologists) to a point on the palate level with the anterior edge of the lachrymal foramen. This foramen is sometimes entirely closed up, but its position can always be easily made out. 4 From the same point on the palate to the tip of the premaxilla?. s Length of brain-case : length of rostrum : : 100 :- 8 Measured with No. 8 shot. The indices Nos. 2 and 5 are of tbe greatest service, as giving a far more exact idea of the proportions of the skull than any mere measurements can do. The " rostral index " is especially useful in the present group, as the relative length of the snout has such an important bearing upon the general form of the skull. Greatest length1. mm. no-o 94-0 Length of rostrum1. mm. 520 40-0 Greatest breadth. mm. 45-0 44-0 Rostral index5. 100 83 Index of breadth-. 40-9 42-5 Interorbital breadth. mm. 14-5 17-0 Length of brain-case3 mm. 52-0 48-0 Capacity6. c.cm. 22 22 |
