476 Prof. Owen on an Incisor of Notothcrlum MItcliellii. 



contracts to an antero-posterior diameter of 1 inch 3 lines, 

 and a transverse diameter of 4^ lines, at the end of which it is 

 excavated by the shallow remnant of the pulp-cavity (fig. 6). 

 The breadth here, owing to the opposite lateral channels, is 

 least at the middle of this end, where it contracts to 3 lines ; 

 the part anterior to this gives the breadth of 4^ lines. Thus 

 the present tooth is less " fusiform '' than Prof. M'Coy's spe- 

 cimen, which may be due to its having come from a less aged 

 individual. The Professor's description of the "crown, worn 

 down obliquely almost to the base, only about an inch of it re- 

 maining," applies, however, equally to my specimen. The 

 cement-covered outer surface of the root is marked by the same 

 " short, irregular, interrupted longitudinal grooves," with inter- 

 vening ridges about a line in breadth. 



The difference between the Professor's specimen and the 

 homologous tooth of Thylacoleo carnifex is, first, in dimensions. 

 The figure given at p. 448, loc. cii., is reduced to nearly half the 

 natural size of the tooth there described, and of the Professor's 

 original " pen-and-ink sketch ;" and a notice of such reduction 

 in the woodcut has been omitted. As it stands, it nearly re- 

 presents the natural size of the upper front incisor of Thylacoleo 

 carnifex, the root of which is about 2 inches in length, and 

 10 lines in greatest breadth. But the crown is relatively 

 longer, the enamel is twice as thick, and its free end is not 

 bevelled off chiselwise, as in Professor McCoy's specimen and 

 in mine of Nototherium Mitchellii. 



Reduction of figures of single or detached teeth should, if 

 possible, be avoided : it detracts much from the facility of com- 

 parison. The figures of the tooth here described are of the 

 natural size. 



I may add that the tooth supposed to be a canine of Thyla- 

 coleo is shown to be an incisor, in my second memoir on that 

 genus in the Philosophical Transactions for 1866. 



I am, Gentlemen, 



Yours faithfully, 



Richard Owen. 



British Museum. 



EXPLANATION OF PLATE XVL 



Tig. \. Side view of anterior incisor, right side, upper jaw, of 'Nototherium 



Mitchellii. 

 Fig. 2. Back view. 

 Fig. 3. End view of crown. 

 Fig. 4. Outer-side view of crown. 

 Fig. 6. Section of middle of root. 

 Fig. 6. Free extremity of root. 



