20 
Proceedings of the Royal Society of Edinburgh. [Sess. 
figured by Monro secundus * who described a concave tympanic membrane 
at the bottom of the meatus ; a communication between the cavity of the 
tympanum and other cavities analogous to the human frontal, sphenoidal 
and maxillary sinuses ; an Eustachian tube which connected the tympanic 
cavity with the nasal chamber. The prolongation of the tympanic 
membrane into the air sinuses in these bones of the skull, as well as into 
the palate bone, has been described by Rappj* in the porpoise, and by 
Claudius l in Delphinus delphis, together with the relations of the ossicles 
to the tympanic membrane and the communication of the Eustachian tube 
with the cavity. 
The Petrous in the large whales is a heavy massive bone interlocked 
with the base of the skull, consisting of three definite divisions, a central 
labyrinthine part which contained the cochlea, vestibule, and semicircular 
canals, in which the auditory nerve was distributed ; a short anterior pre- 
otic process and a long posterior opisthotic process, as an example of 
which Balcenoptera sibbaldi may be described (rig. 3). The Labyrin- 
thine division had a rough upper surface in relation to the basis cranii ; a 
smooth under surface characterised by a large bluntly conical projection, 
which was directed outwards towards the tympanic, but separated from it 
by the cleft in the tympanic bulla. This surface also formed the roof and 
inner wall of the tympanic cavity ; in it was a funnel-like depression in 
which the stapes was situated and was attached by its foot to the fenestra 
ovalis of the cochlea. The inner aspect of the petrous was prolonged and 
perforated by the large canals for the passage to the labyrinth of the 
divisions of the auditory nerve, and by smaller foramina and canals. 
The Pre-otic was an irregular conical mass which projected forwards to 
end in a more or less pointed process, it was continuous with the labyrinthine 
division, and was connected with the tympanic by the anterior pedicle ; it 
occupied a depression in the squamous temporal above the pterygoid fossa. 
The Opisthotic, so-called mastoid, was a long flattened wing-like plate 
continuous with the labyrinthine division, and connected with the tympanic 
by the posterior pedicle. In one of my specimens of B. sibbaldi it was 
17 inches (432 mm.) long, and in another 5J inches (134 mm.) broad. It 
was locked into a groove between the squamous-temporal and the ex- 
occipital. In a Megaptera the greatest length was 235 mm., and the 
greatest breadth 100 mm. ; in B. musculus, 200 by 70 mm. ; in B. rostrata , 
70 by 35 mm. 
* “On Fishes,” p. 45, Edinburgh, 1785. 
t Die Gehorwerkzeuge der Cetaceen, Tubingen, 1836. Die Cetaceen, Stuttgart, 1837. 
J Physiologische Bemerkungen iiber das Gehororgan der Cetaceen , Kiel, 1858. 
