8 THE VOYAGE OF H.M.S. CHALLENGER. 



golds articulated with each other mesially for 4 inches frora the base of each plate in the 

 adult skull, but then diverged, and allowed first the vomer, and then the two superior maxil- 

 laries to appear between them. In the younger skull, the mesial articulation between the 

 two pterygoids was more complete, for the vomer intervened only at the anterior part. 



The posterior nares and the basis cranii had a similar shape and arrangement of bones to 

 what I have elsewhere described in Meso2)lodo7i sowerbyi. The occipital surface of the 

 skull also had a similar form, and the jugal process of the ex-occipitals was separated 

 by a cleft from the lateral elevation of the basi-occipital ; there was little difi"erence in 

 the configuration of these parts in the young and adult crania. In both an extensively 

 ossified falx was situated in the mesial plane of the cranial cavity. 



The general shape of the squamoso-zygomatic part of the temporal resembled that bone 

 in Mesoplodon sowerbyi, but the fossa in front of the petro-tympanic bone was not so 

 smooth, and had an irregularly ridged and furrowed surface in both the adult and younger 

 crania. A curved spur-like process descended in the younger skull from the squamous 

 temporal in front of the petro-tympanic and aided in retaining it in place. In the adult 

 skuU this process was absent (probably broken ofi"), the tympanic buUge had been re- 

 moved and only the left petrous bone was in place. In the younger specimen the mas- 

 toid, tympanic, and petrous portions of the temporal were distinctly difi'erentiated : the 

 petrous part was a separate element, but the mastoid and tympanic were fused together. 

 The mastoid articulated behind with both the jugal process of the occipital and the 

 posterior prolongation of the squamosal, by a broad roughened surface, whilst anteriorly it 

 was continued into the the tympanic by a constricted neck. I have carefully compared 

 the tympanic and petrous bones of Mesoj)lodon layardi with the corresponding bones of 

 the ZijMus cavirostris from Shetland which I described some years ago.'^ In Mesoplodon 

 the inferior surface of the tympanic was one inch in breadth at its posterior end, where 

 it was divided into an outer and an inner lobe by a groove extending forwards from 

 its posterior end, the outer of these lobes was more boss-like and smoother than the inner 

 (PL II. fig. 7). In Zijohius this surface was not bilobed and possessed a ridge extending 

 in the antero-posterior direction (PI. II. fig. 9). The outer surface of the tympanic was 

 deeper in Ziphius than in Mesop>lodon, and the groove on this surface was more 

 vertical and elongated in the latter than in the former. The inner surface in Ziphius, 

 where it turned into the bulla, was more deeply denticulated that in Mesoplodon. 

 The tympanic in Ziphius was somewhat larger than in Mesoplodon, and the same 

 also was the case with the petrous bones. The longest diameter of the petrous bone 

 in Ziphius cavirostris was 2j^ inches, of the adult Mesoplodon layardi 1^%, of the 

 younger specimen the same : the greatest breadth in Zip>hius cavirostris was 1^, in 

 the adult Mesoplodon layardi lj%, and in the younger specimen lj%^ inch. The 



1 Trans. Roy. Soc. Edinburgh, 1872, vol. xxvi. 



