370 A HISTORY OF BIRDS 



quadrangular in form, and in this respect it differs from all the 

 other species to be described. Further, in these Owls there is 

 no asymmetry traceable ; and this is also true, curiously enough, 

 in other cases only specifically distinct from forms which present 

 the most marked asymmetry. 



In the Owls of the Genus Syrnium the aperture of the ear has 

 increased so as to embrace the entire base of the operculum, 

 which, when raised, exposes to view a large portion of the bon}' 

 skull. In the Tawny Owl {Syrnium aluco) (111. 48, C, D) the aper- 

 ture of the left side of the head is considerably smaller than that 

 of the right, which exposes not only the skull wall but also a large 

 portion of the sclerotic plates of the eye, and herein we meet 

 with asymmetry for the first time. In the closely allied Ural 

 Owl {Syrniujii uralense) the ear differs in that asymmetry, if 

 present at all, is barely perceptible. Passing now to the ears 

 of the Genus Asio we find specialisation carried to still further 

 limits. The aperture of the ear now extends downwards and for- 

 wards to the angle of the gape, and circles upwards and forwards 

 to a point above the region of the middle of the eye (111. 48, 

 A, B). The operculum has now attained a relatively enormous 

 size, and when turned forwards upon itself, reveals the greater 

 part of the skull, and the sclerotic region of the eye, and the 

 posterior half of the mandible. From the middle of the operculum 

 there runs backwards a fleshy valve dividing a spacious semi- 

 lunar depression — bounded in front by the eye, and behind by the 

 post-orbital process — into two moieties differing somewhat in 

 shape on the two sides of the head. But the chief point of 

 interest about the cavity divided by this valve, is the fact that, 

 on the left side of the head the passage to the middle ear opens 

 in the floor of the chamber lying above the valve, while on the 

 right side it will be found in the floor of the chamber belozv the 

 valve and at the base of the lower jaw. 



So far the modifications described are confined entirely to the 

 plications of the skin, but in Tengmalm's Owl {Nyctala teng- 

 malmi) the skull itself presents some important structural 

 changes (111. 48, G, I, H). The membranous portion of this 

 region, which we will examine first, differs only on the two sides 

 of the head in that the left aperture is somewhat larger than the 

 right. The operculum is wanting. Within this aperture, how- 

 ever, there will be found the most unexpected dissimilarity in the 



