Pterylography of Photodilus. 45 



immediately above the articulation of the mandible with the 

 quadrate, its long axis pointing obliquely backwards. It 

 leads into a spacious chamber extending upwards and back- 

 wards for a considerable distance. The passage to the middle 

 ear lies at the bottom of this chamber. 



Immediately above the mouth of the external ear lies a deep 

 fossa (/.). This fossa is entirely open laterally, but is bounded 

 in front by the eye, behind by the post-aural section of the 

 peripheral disc-feathers, above by the base of the post-orbital 

 process, and below by a thin fold of skin, which serves also 

 as the roof of the external aperture of the ear (see PI. II. 

 fig. 2). It appears to correspond to the diverticulum of 

 Asio accipitrinus. 



On the left side (PI. II. fig. 1) the aural aperture will be 

 found to occupy a position exactly corresponding to the 

 position of the diverticulum of the right side, whilst the latter 

 is represented by a very shallow trough or groove lying im- 

 mediately below the aural aperture. The superior angle of 

 the mouth of this aperture will be found to lie on a level with 

 a line drawn backwards and very slightly downwards from the 

 posterior canthus of the eyelid, whilst its long axis is nearly 

 at right angles to the long axis of the skull. The extension 

 upwards and backwards of the aural chamber is much less 

 than in its fellow of the opposite side. The chamber, indeed, 

 on this left side leads almost directly downwards to the 

 tympanum. 



If a comparison be made between the apertures of the ears 

 of Photodilus and of Asio, a point of considerable interest 

 and of not a little significance, will at once become apparent. 

 This is that the relation between the diverticulum and the 

 aperture on the two sides of the head is the same in 

 both genera. That is to say, in both genera the diverti- 

 culum will be found above the aperture on the right side, 

 below it on the left. This suggests that the fold of skin 

 which divides the diverticulum from the aural aperture on 

 the two sides of the head in Photodilus is the homologue of 

 that more extensive fold performing the same office and 

 running from the operculum backwards in Otus. 



