420 PROF. W. H. FLOWER ON THE GENUS MESOPLODON. 
The tympanic bone of Mesoplodon is quite different from that of Ziphius’, the groove 
between the lobes at the posterior end being very well marked as in the true Dolphins, 
whereas in Ziphius it is obsolete. In Hyperoodon also it is scarcely apparent, while in 
Berardius it is nearly as well marked as in Mesoplodon. Thus, by the form of this very 
characteristic bone, Zphius allies itself to Hyperoodon, and Mesoplodon to Berardius, 
and the two former approach nearest to Physeter, and the latter to the less-modified 
Dolphins. 
The relationship between Berardius and Mesoplodon is undoubtedly close. One of 
the specimens (J/. hector’) now under consideration has been placed sometimes in one 
genus and sometimes in the other; and it does certainly present some transitional cha- 
racters; but as it is only known by the skull of a very young individual, it is scarcely 
safe to decide its position, except provisionally, especially as it is not yet known in 
which direction the alterations which must take place during the progress towards 
maturity tend. 
The animal stranded at Lyall Bay, which I have called M. australis, is perfectly 
adult, as shown not only by the union of the sutures at the base of the cranium, 
but also by the condition of the other bones of the skeleton, the terminal epiphyses 
of the bodies, even of the thoracic vertebra, being completely consolidated. The 
mesorostral or mesethmoid cartilage is densely ossified throughout its whole length. 
There is still much to be learned with regard to the mode of ossification of this cartilage. 
All the specimens which I have had an opportunity of examining are either so young 
that ossification has not commenced, and the trough of the vomer in the rostrum 
proper is completely empty in the dried skull, or so old that the consolidation of the 
cartilage and its union with the surrounding bone has been completed. But it must 
be observed that, although the cartilage appears to be nothing more than a con- 
tinuation forwards of the ordinary mesethmoid lamina or septum of the nose, the 
ossification is not a simple extension forwards of that which occurs in all Cetacea (in 
all Mammalia, in fact) in the hinder or internarial portion of the septum, but appears 
to be an independent production, peculiar to the genera Mesoplodon, Ziphius, and certain 
allied extinct forms. It is separated by an interval (which appears to diminish with age, 
but of which traces can always be seen on the upper surface of the rostrum near its 
base) from the true mesethmoid ossification. It differs from the latter in being 
intensely hard and compact, whereas the mesethmoid is, especially at its anterior part, 
somewhat spongy in texture. It differs also in showing strong indications of being 
formed by a pair of lateral ossifications, united in the middle line, as the upper surface 
in many parts and the anterior apex show a marked median groove. I think it will 
be well therefore to adopt Professor Turner’s name of “ mesorostral” bone for this 
solid bar forming the centre of the rostrum, restricting “ mesethmoid” to the part 
lying between the nares and a short distance in front of them, which is ossified in the 
* Figured by Prof. Turner, loc. cit. plate xxx. 
