PBOFESSOE OWEN ON THE FOSSIL MAMMALS OF AITSTBALIA. 533 
nearly straight ; the very slight degree in which they deviate from that line tends to an 
upward curve (Plate XXXIX. fig. 4). 
The length is 10 inches, the circumference 5 inches 6 lines. The longitudinal ex- 
tent of the worn surface in those of the skull (Plate XXXV.) is 3 inches ; its transverse 
breadth is 1 inch 4 lines. The transverse section of the entire tooth (Plate XXXIX. 
fig. 6) is oblong; in some it presents an irregular oval with the small end upward. The outer 
side at its lower two-thirds is usually prominent ; the inner side is more even or flat, in 
some instances feebly convex ; in one specimen very slightly concave along its middle third. 
The outer side is more constantly traversed by a narrower shallow longitudinal channel, 
rather above the middle of that side. The enamel (Plate XXXIX. figs. 5, 6, e) is con- 
tinued from the border of this channel round the lower part of the incisor, to about one- 
fifth of the extent of the inner side (ib. e ') : its terminal borders are abrupt on both sides, 
with the rather thick cemental covering of the unenamelled part of the circumference 
extending over the enamel borders. The surface of the enamel is finely ridged length- 
wise and reticulo-granulate ; the minute studs of enamel being, however, more conspi- 
cuous than the holes ; although these are not absent. 
About two-thirds of the tooth is lodged in the socket, which extends backward a little 
beyond the symphysis, but without causing, as in Rodents, a prominence of the inner 
wall of the ramus (Plate XLI. fig. 2) ; in this respect Biprotodon resembles Macropus 
and Phciscolomys. The line of the socket forms an angle of 147° with the basal line 
of the mandibular ramus. The pulp-cavity (Plate XLII. fig. 5, p) is a long cone 
widely open at the base. The pair of tusks run almost parallel, slightly approximating 
so as to come into contact at their working ends. 
The form of the lower incisor, described as it is shown in the most perfect specimen 
of the lower jarv of a full-grown example, is subject to some variety. Being a tooth of 
unlimited growth, it increases with the size of the jaw. In young specimens the out- 
swelling of the outer side, or the contraction of the upper third of that side, is either not 
apparent or not so conspicuous, and the transverse section of the incisor yields a full 
oval, as in that of the young Biprotodon from the Wellington Valley Cave * (Cut, fig. 1 a), 
and, slightly modified, in the one of similar age from Darling Downs, Queensland (Plate 
XLI. fig. 1, a). 
But under all these slight varieties, which I cannot regard as specific, there prevail the 
same essential characters of structure, disposition of enamel, &c., pointed out in my 
original Memoir as differentiating Biprotodon from Halichore , Hippopotamus , and other 
Mammals with tusks of similar size. 
A diastema, between three and four inches in extent, rises gently as it recedes from the 
incisor (Plate XLI. 7), to the first molar ( d 3), and more so, as the molar series becomes 
completed and pushed out for use as in Plate XXXV. fig. 1, and Plate XLII. fig. 2. 
Of the first molar tooth ( d 3) I have no specimen. Its existence was indicated by traces 
of its socket in the portion of mandible obtained by Dr. E. C. Hobson, from a gravel- 
* Mitchell’s ‘ Three Expeditions into the Interior of Australia,’ 8vo, 1838, vol. ii. p. 362, pi. 31. figs. 1 & 2 . 
