516 CETACEA. 



The oiDening for the aqueduct of Eallopius looks along the groove of the 

 posterior process, and with the fenestra ovalis Kes as usual in a hollow in which 

 they are separated from each other hy an interval of only 1-03 of an inch, the 

 former heing the most internal, looking outwards and downwards. Anterior 

 and slightly external to these is the almost circular hoUow for the head of the 

 malleus. 



Cochlea is flattened on the under or apical surface, somewhat globular in 

 form and relatively of large size compared with the semi-circular canals, PI. XL, 

 figs. 1 and 2. Immediately posterior to the internal opening of the aqueduct of 

 Eallopius, there is the orifice of the canal of the vestibule opening into the anterior 

 wall of the vestibule, unmediately above the external margin of the fenestra ovahs. 

 In close relationship with the internal auditory meatus is the aqueduct of the 

 cochlea, which is rather large with a wide internal orifice. 



The aqueduct of Eallopius has a length of 0"-24 and is curved from above 

 downwards and backwards, near its termination having a slight inward curve. 

 Near its lower end its anterior wall is marked by a depression perforated by 

 the minute canal which probably transmits the large, superficial, petrosal 

 nerve. 



The fenestra ovalis is transversely oval, and within its outer and inner margins 

 there is the rim on which the stapes rests. The vestibule is 0"'16 in length and is curved 

 upwards, backwards, inwards and downwards, but its two extremities are nearly on 

 the same level. Its inner wall, immediately above the fenestra ovalis, is deeply con- 

 cave, while the portion posterior to it is more contracted, the inner wall terminating 

 in the commencement of the lamina spu'alis which curves backwards and outwards. 

 In the anterior extremity of its outer wall, there is a deep pit, in the bottom of 

 which is the anterior end of the superior semi-circular canal, and in the floor of 

 the vestibule immediately internal to the fenestra ovalis is another depression con- 

 tinuous with the former, but at a considerably lower level, and in wliich is the 

 anterior extremity of the external semi-circular canal. In the roof of the anterior 

 extremity of the vestibule the canal of the auditory nerve opens, being immediately 

 posterior in position to the aqueduct of Eallopius. The vestibule is rather highly 

 arched at the middle of its roof, into which opens the minute aqueduct of the ves- 

 tibule, transversely to the long axis of the vestibule, protected below by an inwardly- 

 projecting shelf of bone. The posterior extremity of the vestibule bends down- 

 wards and inwards to the scala vestibuU as a narrow tube compressed from above 

 downwards. 



In the fenestra rotunda which has the position this opening occupies in 

 Cetacea, the ridge corresponding to the canal described by John Hunter^ as peculiar 

 to tins group of mammals is distinctly visible, in addition to the ridge of the 

 scala vestibuU which is the uppermost of the two. This secondary canal appears 

 faintly as a ridge to the left of the canal of the cochlea in fig. 1, PL XLI, also in 

 OloUcephalus and Monodon, figs. 7 and 10, close to the protuberance corresponding 



» Phil. Trans., 1787, p. 436. 



