856 AMPHIBIA. 



and the region superior and anterior to the ex-occipital as the pro-otic ? This view 

 would seem to be supported by an examination of the relation of the constituent 

 parts of the capsule. The exoccipital lies wedged in between the two elements. 

 It is just possible that the sides of this part of the cranial cavity may be formed 

 by the pro-otic, but the apparent overlapping of the exoccipital by the pro-otic 

 appears to negative this view of their relations. The position of the fenestra ovalis, 

 and the rudiments of the stapes and of the semi-circular canals and other important 

 structures, would appear to establish the foregoing opinion regarding the nature of 

 this auditory capsule, and which will now be described in detail. 



Fenestra ovalis. — The general direction of this structure is backwards and out- 

 wards, and it is placed somewhat external to the exoccipital condyle, from which it 

 is separated by the foramen of the 8th pair of nerves. It is a short cylinder with 

 a distinctly rounded orifice, closed by a membrane containing a smaU bony nodule, 

 evidently the stapes. The inner surface of the lower wall of the tube is on a level 

 with the floor of the auditory cavity, and the internal opening is placed a little 

 behind the centre and immediately below the inner opening of the semi-circular 

 canals which lie in the roof of the chamber. 



Semi-circular canals. — These structures can be detected on the external surface 

 of the roof of the chamber as two rounded ridges, and their presence can be at once 

 demonstrated by a vertical section of the capsule, or by cutting the roof away 

 gradually (fig. 31 c). They are on the same level and close to the roof of the 

 chamber. Their construction is very simple. A flat process arches in from either 

 side of the chamber to the roof, and both meet in the centre. A canal is thus 

 formed between the upper surface of each process and the roof. The floor of the 

 anterior canal is not so broad as the posterior one, which is slightly curved outwards 

 and downwards and forwards, while the former is simply forwards and outwards. 

 Their external openings are on a line with the external border of fhe fenestra ovalis, 

 and their whole length occupies about the middle half of the total transverse length 

 of the chamber. Their external openings overlook a crypt-like depression or sacculus 

 in the outer extremity of the chamber, which probably represents the cochlea. On 

 a vertical section of the dry capsule, its cavity is seen to have been filled with a 

 pure snow-white friable substance resembling finely-ground chalk invested by a 

 delicate membrane. In a specimen which had been preserved in alcohol, it is a 

 shrivelled and contracted mass suspended in the centre of the chamber from the 

 internal openings of the semi-circular canals. The sac is doubtless the equivalent 

 of the utricle, and it is probable that the white contents are calcareous. 



On the under surface of the capsule, external to the parasphenoid ridge, there 

 are three, or sometimes more, small foramina on the anterior and inner side of the 

 inferior pterygoid process. Two of them communicate and open externally, but the 

 other passes through the substance of the floor of the capsule and opens into the 

 cranial cavity close to a foramen, which appears to transmit the acoustic branch of 

 the 7th nerve. If this is the course of the facial nerve, we have a very clear indi- 

 cation of the homologies of this capsule. The anterior extremity of the pro-otic 



