CETACEA—LILLIE. 107 
It consisted of a firm case of yellow elastic tissue (D), which enclosed a loose 
fibrous layer, heavily charged with fat (C). The outer coat (EK) was formed of 
spongy tissue containing air-spaces, and attained a thickness of some 4 inches 
or more. The above-mentioned caps of tissue were underlaid ventrally by the 
muscular wall of the pharynx (F). The tympanic cavity was thus separated from the 
cavity of the pharynx by very nearly a foot of tissue. The tympanic cavity and air- 
spaces, such as the pterygoid fossa, were the same as in Balaenoptera, The air-passage 
(see text-fig. 10, B-G) which opened on the dorsal wall of the naso-pharynx, close to 
the inner extremity of the nasal septum, and communicated with the pterygoid fossa, 
was termed by me the Eustachian tube, in the description of the auditory apparatus of 
Balaenoptera.* This tube should, however, be called the pterygoid fossa tube, as the 
opening of the Eustachian tube proper was situated in a shghtly more posterior 
position, on the ventral wall of the naso-pharynx, near the free edge of the velum palati. 
The Eustachian tube, which was very much wider than the air-passage supplying 
the pterygoid fossa, led directly to the tympanic cavity, and also supplied air to the 
coat of spongy tissue surrounding the bulla. 
The question naturally arises: how do sound-waves reach the labyrinth in 
whales? In the smaller Cetacea, such as Phoeaena and Monodon, the external meatus 
is open, and the tympanum is concave externally, as it is in other mammals. It would, 
therefore, seem possible that in these whales the sound-waves are transmitted to the 
middle-ear through the water in the meatus.f 
But in the larger forms, such as JJegaptera, the meatus is closed up, and there is 
no way for sound-waves to reach the ear through the mouth. 
It is possible, however, that these whales hear through their nasal passages. 
Sound-waves could be conducted through the water to the alar fibro-cartilages, which 
form the elastic lids of the external nares. Within the blowholes there is a continuous 
column of air, leading into the tympanic cavity through the Eustachian tube. Sound- 
vibrations could be transmitted through this air to the walls of the dense, cowrie-shaped 
bulla, which is attached to the periotic by two slender pedicles, and is surrounded by 
air-containing tissue. The bulla should, therefore, act as a sounding-box, and it is 
connected to the fenestra ovalis by the chain of ossicles. 
It is also possible that sound-waves are conducted from the alae of the external 
nares to the bulla, by means of the nasal septum. In this case the mechanism of 
audition in the larger Cetacea would resemble, to some extent, that of the diving 
Mosasaurians, such as Plioplatecarpus,f although the parts involved are formed out of 
very different structures morphologically. The fibro-cartilaginous covers to the nostrils, 
together with the attached cartilage of the nasal septum, which occur in whales, would 
* Lillie, D.G., Proc. Zool. Soc., 1910, p. 779. 
+ Denker, A., Anat. Hefte, Wiesbaden, 1902, Abt. 1, Bd. XIX., Heft. LXII., pp. 423-447. 
t Dollo, L., Bull. Soc. Belge Géol., Bruxelles, Tome XTX., 1905, p. 125, pl. IIT. 
