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



tympanies of any of the Balcenidce that have come under my observation — not more 

 indeed than about one-third the size of the corresponding bones of the Balcena australis. 

 The British Museum, the Museums of the Royal College of Surgeons, and the University 

 of Edinburgh, do not contain any specimens similar in size and form, or marked with 

 similar furrows. Whether they belong to existing species or to some extinct cetacean 

 must for the present be left undetermined, though, from their scanty coating of manganese 

 as compared with the thick covering possessed by others of the bones, obtained from tlie 

 same station, it is not likely that they were so ancient as the more thickly coated forms. 



The third group consisted of eight buUse, from 2^ to 3 inches in length. Only one 

 specimen, which was unfortunately not entire, could be freed from manganese, so as to 

 enable one to see its form. It was 2 '6 inches long. It had a certain similarity in form 

 with the tympanic bone of the ZiiMus cavirostris referred to in a former part of this 

 report as having been obtained from Shetland. It differed from it, however, in several 

 particulars. It was not only longer, but generally more massive, and did not possess 

 the somewhat unciform lobe at the posterior end of the under surface seen in Zijjhius 

 cavirostris (PL II. fig. 12) ; on the outer surface a faint concavity extended in the 

 longitudinal direction instead of being slightly convex, as in cavirostris. The inferior 

 surface, again, was more rounded, and not raised into a feeble roughened ridge as in the 

 Shetland specimen (PL II. fig. 9). For the present this bone must be left undetermined. 



The fourth group consisted of about forty specimens, generally from 1'6 to 2 inches 

 long, although one was 2'3 inches, and another 2'2 inches. In all, the posterior end of 

 the under surface had the bilobed character of the bulla. The two largest specimens 

 (PL VIII. figs. 1, 2, Mr Murray's Report) were compared both with Professor Flower's 

 figures of the petro-tympanic bones of Berardius arnouxii^ and with the bones them- 

 selves, to which they approximated somewhat in size. They differed, however, from the 

 tympanic bullse in that animal in having the external posterior lobe more boss-like, and 

 in possessing a much wider furrow between that lobe and the internal j)osterior lobe ; 

 moreover, the inferior surface in them was not so irregularly roughened as in Berardius. 

 In appearance they corresponded closely with the tympanic bullae of Meso-plodon layardi, 

 described in an earlier part of this report, but they were on a larger scale, for not only 

 was the larger of the two bullae half an inch longer; but its greatest width was 1'5 

 inch, whilst that of Mesojdodon layardi AA'as 1"3 inch. They should, I believe, be referred 

 to the genus Mesoplodon, though probably to a larger species than layardi, for although 

 my specimen of Mesoplodon layardi was from an immature animal, yet the petro-tympanic 

 bones in the Cetacea assume their full dimensions at a comparatively early period of life. 

 In both these specimens the j^etrous bone was united with the tympanic. It closely 

 resembled in shape that of layardi, but was somewhat bigger, for its length was 2 '2 

 inches, whilst that of layardi was 1'9 inch. 



' Trans. Zool. Soc, vol. viii. 



