REPORT ON THE CETACEA. 9 



internal meatus of the petrous bone was a single canal in the young Mesoplodon, but was 

 divided into two in the adult Mesoplodon and in Zipliius. The surface anterior to the 

 meatus was roughened and slightly convex in Mesoplodon, but was elevated into a 

 prominent tubercle in Ziphius. In both, the stapes formed a solid column of bone 

 aukylosed to the inner wall of the tympanum (PI. I. fig. 5). The most important difference 

 between the petro-tympanic bones in the two animals was the bilobed character of the 

 under surface of the tympanic in Mesoplodon and not in Ziphius, a character which Meso- 

 pilodon layardi shares with the other species of Mesoplodon described by Professor 

 Flower in his recent memoir on this genus/ which is possessed by the true Dolp>hins and, 

 as he points out, also by Berardius. Thus, by its tympanic bone, Ziphius may be dis- 

 tinguished from Mesoplodon as readily as by the differences in the naso-premaxillary 

 region, the value of which I dwelt on in my former memoir on these genera." 



In Mesoplodon layardi the sphenoid took but a very small part in the formation of 

 the temporal fossa. The parietal formed the larger part of its floor, and in the younger 

 skull could be followed as a distinct bone situated between the supra-occipital and the 

 frontal to the vertex, where it was united by synostosis to its fellow. In the adult, though 

 the outline of the parietal in the temporal fossa could be readily seen, no part could 

 be traced beyond the fossa to the vertex, for it was overlapped by the growth of the 

 supra-occipital, so that only the thin edge of the frontal bone appeared in the interval 

 between the supra-occipital and superior maxillary. The vertex part of the frontal 

 articulated anteriorly, as in Mesoplodon sowerhyi, with the superior maxiUse, prsBmaxillse, 

 and nasals. The frontal formed the roof of the orbit and possessed a strong postorbital, 

 but a feeble preorbital process. The malar bone consisted anteriorly of a flattened plate, 

 which articulated Avith both the superior maxilla and the lachrymal : from this plate a 

 long slender zygomatic bar passed backwards below the orbit to articulate with the 

 zygomatic part of the temporal. The lachrymal closely resembled in shape the corre- 

 sponding bone in Mesoplodon soiverhyi; in these skulls it articulated anteriorly and 

 externally with the preorbital process of the frontal, the malar, and the superior maxilla. 



The mandible was absent in specimen A, only its anterior part was preserved in B, 

 whilst in C, tkough both halves were present, the condyloid ends were much broken. In 

 C the right and left halves were not ankylosed at the symphysis, which part of the bone 

 was 4^ inches long. In B the union between the two halves was complete, and the 

 symphysis was 11^ inches long. 



During the time that the ChaUenger was in the harbour of Wellington, New Zealand, 

 IVIr Moseley visited the Wellington Museum, and made a careful comparison between the 

 lower jaw and teeth of specimen B, and the jaw and teeth from the Chatham Islands 

 preserved in that Museum, which have been described and figured by Dr Hector. 



1 Trans. Zool. Soc, vol. x., 1878. = Trans. Roy. Soc. Ei.lin., vol. xxW., 1872. 



(ZOOL. CHALL. EXP. — PART IV. — 1880.) D 2 



