476 Prof. Owen on an Incisor of Nototherium Mitchellu. 
contracts to an antero-posterior diameter of 1 inch 3 lines,’ 
and a transverse diameter of 44 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 44 limes. 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 mter- 
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, doc. cit., 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 M‘Coy’s specimen and 
in mine of Nototherium Mitchelli. / 
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. 
{ am, Gentlemen, 
Yours faithfully, 
Ricuarp Owen. 
British Museum. 
EXPLANATION OF PLATE XVI. 
Fig. 1. Side view of anterior incisor, right side, upper jaw, of Nototherium 
Mitchellit. 
. Back view. 
. End view of crown. 
. Outer-side view of crown. 
. Section of middle of root. 
. Free extremity of root. 
Fig. 
Fig. 
Fig. 
Fig. 
Fig. 
Or to bo 
