1885.] MR. O. THOMAS ON THE RACES OF ECHIDNA. 333 



these being, in the absence of teeth, the only characters available for 

 conipaiison. 



With regard to age the following are the measurements of two 

 skullSj both of var. aculeata, and both from Central Queensland : — 



Greatest Greatest Index of Length of 



length'. breadth. breadth-. brain-case', 

 mm. mm. mm. 



i'. Adult. . . . 110-0 4.5-0 40-9 520 



/. Young . . 94-0 44-0 42-5 48-0 



Length of Rostral Interorbital ^ ■, a 



rostrum'. index'. breadth. ^' ^■ 



mm. mm. c.cm. 



I. Adult. . . . 520 100 14-5 22 



/.Young .. 40-0 83 1/-0 22 



Taking these two as examj)les, we see that young skulls have com- 

 paratively large rounded brain-cases, short snouts, and broad inter- 

 orbital 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 U[), 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. Alto- 

 gether, 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 compa- 

 rison of a considerable series that any satisfactory estimate of ai;e 

 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, 

 sucli as r', «', 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.seiosa that I have seen have open 

 vacuities, while without exception the long-spined northern exam))les 

 hare 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- 



' From the tip of the premaxillse to the most posterior point of either 

 condyle. 



^ Length : breadth : : 100 :— 



' From the centre of the lower edge of the foramen magnmn (lasion of an- 

 thropologists) 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. 



' From the same point on the palate to the tip of the premaxiUee, 



•' Length of brain-case : length of rostrum : : 100 : — 



" Measured with No. 8 shot. 



The indices Nos. 2 and 5 are of the greatest service, as giving a far more exact 

 idea of the propoi'tions 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. 



