STRUCTURAL ADAPTATIONS 371 



nature of the bony regions. Thus, on the left side of the head 

 (111. 49) the combined post-orbital process and tympanic wing 

 of the exoccipital forms a large outstanding shield extending 

 downwards to the level of the lower jaw so as to leave a narrow 

 chink between itself and the bony ring of sclerotic plates sur- 

 rounding the eye (111. 49, top left-hand figure). The chink 

 traced inwards gives way to a large chamber, in the floor of 

 which will be found the passage to the middle ear. On the right 

 side this chamber is wholly exposed but for a tongue-shaped 

 plate of bone which, extending forwards to the bony rim of 

 the eye, encloses the lower half, leaving a small triangular space 

 lying immediately above the lower jaw (III. 49, top right-hand 

 figure). 



The feathers which form the periphery of the curious facial 

 discs of these birds — and especially of the Barn Owl, Tawny 

 and Long and Short-eared Owls — play no small part, probably, 

 in the collection and transmission of sound waves to the middle 

 ear. In the Barn Owl these peripheral feathers are long, narrow, 

 curved forwards, and closely set, forming during life a trumpet- 

 shaped wing, such, for example, as one makes by placing the 

 hand to the ear when endeavouring to catch indistinct sounds. 

 Near the centre of the concavity of this rim, as will be seen in 

 the figure, the aperture of the ear is placed. The arrangement 

 of the peripheral feathers in the Tawny Owl does not differ 

 essentially from that of the Barn Owl. In the Long and Short- 

 eared Owls {Asionidce) these feathers are arranged so that those 

 of the uppermost segment slope downwards, while those in the 

 lower segment of the rim slope upwards. So closely are the 

 bases thereof packed that the supporting skin fold is drawn out 

 so as to stand far away from the head, and so further increase 

 the sound-catching trumpet. 



The function of the operculum is by no means clear. 

 Primarily, it seems to serve as a protection to the wall of the 

 skull, but in the living bird it is evident that if it were slightly 

 raised or swung outwards, much as a door on a hinge, the 

 feathers along its free edge would, in conjunction with the peri- 

 pheral disc feathers, form a sort of laterally compressed or 

 vertically slit-shaped mouth resembling the semi-tubular ear 

 conch of many mammals. By such an arrangement sounds 

 can doubtless be much more easily located than is possible 



