PROF. W. H. FLOWER ON THE GENUS MESOPLODON. 43: 
or 
differences by which the group may be divided into two sections, not equivalent to 
those of Gervais, but to which, if thought advisable, his names might be applied, as 
his Mesoplodon sowerbiensis may be taken as the type of one, and his Dioplodon 
densirostris as the type of the other. To the first (JZesoplodon proper) belong M. bidens, 
M. europeus (if distinct), MW. layardi, and M. hectori; to the second (to which the 
name Dioplodon may be given if it should appear that a generic term is necessary 
to distinguish it), 1. densirostris, M. australis, M. grayi, and M. haasti. Certain cranial 
characters, and the position of the large mandibular tooth, distinguish J. hectori, and 
may possibly afford grounds for placing it in another section intermediate between 
Mesoplodon and Berardius; but with the present limited knowledge of its structure, 
derived only from the cranium of a very young individual, I think it preferable to 
retain it provisionally in the position to which it is assigned above. The structure 
of the vertebral column, when known, will throw light upon its relationship to 
Berardius. The objection that the name Mesoplodon is not applicable to this species 
is scarcely worth considering, as it is a case in which “the sooner a term becomes an 
arbitrary sign the better” }. 
The further subdivisions which have been propesed, as Dolichodon and Callidon, of 
Gray, and Oulodon of Haast, appear to me unnecessary, though, with regard to the 
latter, this may be a matter of opinion. 
It has been assumed above that the different individuals assigned by various authors 
to the I. bidens or sowerbiensis are really one species; for I have not had an opportunity 
of making a critical comparison of them, though it would be desirable to do so as soon 
as a sufficient number of specimens are collected in our Museums to afford materials for 
thoroughly estimating all the variations to which they may be subject according to sex 
and age. Perhaps more light may then be thrown upon the relation to that species of 
the solitary example known of MW. ewropeus, which is at present not a very satisfactorily 
established species. 
Of the southern species, 7. layardi, M. densirostris, and M. grayi are thoroughly 
well established; complete skeletons of each are in existence, and have been more or 
less perfectly described, though, unfortunately, no skeletons of either of the two former 
have yet reached Europe. JZ. australis is known by the skeleton, complete but for the 
important exception of the mandibular teeth; further evidence of its distinctness from 
M. grayi may perhaps be considered desirable. J. haasti is only known by a rostrum 
and mandible. J. hectori must be a totally distinct form, though at present very 
imperfectly known. 
The geographical distribution of the group has a very great interest in relation 
to that of many other Australian groups, both of vertebrates and invertebrates. 
Among the earliest known remains of Cetacea, in the Belgian and Suffolk Crags, Mesop- 
lodon and closely allied forms are most abundant. Up to little more than ten years 
? Owen, Trans. Geol. Soc. 1857, p. 55. 
