36 ON AN EXTINCT MONOTREME, 
parable with the existing platypus must have seemed a mere 
matter of time and good-hap; and by time and good-hap the 
expectation has been fulfilled, and a relic has been found of an 
ancestor of the living ornithorhynchus. The bone, a right tibia 
of an adult, has been very lately received from King’s Creek, 
in the vicinity of Pilton, where the Museum collectors were for 
a short time excavating. It shews no sign of having been 
inherited from a less modified, that is more reptilian, precursor ; 
on the contrary, it possesses all the character of the genus as 
represented by paradoxus, fully matured and even more pro- 
nounced than in its descendant. It is, perhaps, worthy of 
remark that, presuming this tibia to be full-sized as well as 
adult, it indicates a species of smaller dimensions than the present 
one. If, then, the extinct species were the only one then existing, 
it formed an exception to the general rule, which maintained 
superiority of size in members of every group, compared with 
that of their modern representatives. It may not, therefore, be 
too rash to infer that the customary giant of its tribe has yet 
to make itself known. 
Viewed in common with a recent bone of paradoxus, the 
specific distinctness of the fossil tibia is seen at a glance—a 
closer examination leaves, for the moment, a doubt on the mind 
whether its owner were, strictly speaking, an ornithorhynchus 
or of a genus nearly allied. The feature, which has been noted 
by Sir R. Owen as one of those distinguishing the tibia of 
ornithorhynchus from that of echidna, the curvature of the 
shaft, is in the fossil exaggerated, and the whole surface is more 
deeply impressed and sharply moulded by the muscles than in 
the living platypus. It is this circumstance which has suggested 
the specific name agilis. 
The comparative measurements of the tibia in paradorus and 
agilis are these :— 
