PKOE. TV. H. FLOWER ON THE GENUS MESOPLODON. 435 



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 (Mesoplodon proper) belong M. bidens, 

 M. europmis (if distinct), M. 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), M. densirostris, M. australis, M. grayi, and M. haasti. Certain cranial 

 characters, and the position of the large mandibular tooth, distinguish M. hectori, and 

 may possibly afford grounds for placing it in another section intermediate between 

 Mesoptlodon 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 " l . 



The further subdivisions which have been proposed, 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 M. 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 M. europtceus, which is at present not a very satisfactorily 

 established species. 



Of the southern species, M. 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. M. 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. M. haasti is only known by a rostrum 

 and mandible. M. 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 

 1 Owen, Trans. Geol. Soc. 1857, p. 55. 



