38 ON AN EXTINCT MONOTREME, 
The sum of the differences observed would almost seem to be 
beyond the limits of specific variation; but on the evidence of 
this fossil alone it would be imprudent to propose a new genus 
for it. Other portions of the skeleton will, however, be sought 
for with increased interest. 
Since the foregoing notes were made, a mandible (Fig. 3) has 
come to hand from the same spot as that which yielded the tibia. 
Both bones are of the same dark colour, and in the same state of 
mineralization. They, therefore, probably belong to the same 
individual. The mandibulary fossil is the distal half of the right 
horizontal ramus with the colander-like socket of the molar 
nearly perfect. In accord with the tibia, it shows a smaller and 
slenderer animal than paradowus It is narrower in proportion 
to its length, and especially narrower in the postalveolar region, 
Other specific differences are patent in the arrangements of the 
perforated depressions and subdivisions of the alveolus. In the 
recent species, the pits in which are moulded the mammillary 
processes of the under side of the horny grinder are disposed in 
three groups, separated by low septa. The anterior of these 
contains two pairs of depressions, and one or two smaller sub- 
sidiary pits. The middle group consists also of two pairs of 
pits; and the third, confined to the posterior angle of the 
alveolus, of a single depression. In the fossil there are four 
groups, divided from each other by transverse ridges. The fore- 
most contains two pits, the second also two, the third four, and 
the fourth a single pit. There are no subsidiary pits in the first 
of these, and the arrangement of the whole has more lnear 
regularity and lateral symmetry than in the living species. The 
internal coronoid or pterygoid process is well developed. The 
inner angle of the jaw is rather more distinctly inflected than in 
the present representative of the genus. On the whole, nothing 
can be observed in this mandible to confirm the suspicion pre- 
viously expressed that the extinct monotreme was something 
other than an ornithorhynchus. 
