420 PEOF. W. H. PLOWEE ON THE GENUS MESOPLODON. 



The tympanic bone of Mesoplodon is quite different from that of Ziphius 1 , 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 whereas in Ziphius it is obsolete.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, Ziphius 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 (M. hectori) 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 vertebrae, 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 

 1 Figured by Prof. Turner, he. cit. plate xxx. 



