STRUCTURAL ADAPTATIONS 369 



part of the body on the ground, as in the short-necked kan- 

 garoos, for example. 



Perfect bilateral symmetry is almost universal among birds, 

 but there arc a (ew remarkable exceptions to this rule. Save 

 in a single instance — the case of the Wry-billed Plover — these 

 exceptions — afforded by certain Owls — ^are all superficially 

 symmetrical. That is to say, the general contour of the bird 

 is not affected thereby, and the asymmetry is only apparent 

 after careful examination. In this respect, of course, it resembles 

 the intestinal convolutions, for example. Nevertheless, it is not 

 to be included in the same category, neither is it so easily ac- 

 counted for as this intestinal coiling. On the contrary, it presents 

 us with a problem so far unsolved, though not, perchance, in- 

 soluble. 



The Owls to which reference has just been made are asym- 

 metrical in respect to the external ear, and it will be necessary, 

 for a proper understanding of this remarkable phenomenon, to 

 describe certain other peculiar characters associated with, but, 

 be it noted, by no means essential to, this asymmetry. In no 

 bird is the aperture of this organ provided with a fleshy concha 

 such as is found in the Mammalia : that is to say, they are 

 earless in the ordinary sense of the term. The ear of the bird, 

 as in the reptile, opens externally, on the side of the head, by 

 a small aperture, which in the bird at least is almost invariably 

 round, and, save in vultures and some few other exceptions, it 

 is concealed by the slightly modified feathers known as the 

 ear-coverts. In the Eagle Owls {Bubo) this aperture becomes 

 much enlarged, and is oblong in form. In the Genus Scops this 

 increase is still more marked, so much so, that the sclerotic plates 

 encircling the eye become exposed (III. 48, F). Behind the eye 

 there is found a large cavernous chamber at the bottom of 

 which lies the passage to the middle ear. The modifications 

 which we are now to describe may well have had their origin 

 in the variations in the direction of increased size which the 

 members of these two genera have preserved. One of the most 

 striking features in these transformations is the development of 

 a membranous fold or flap from the anterior border of the 

 aperture to form an operculum or cover therefor. In the 

 Strigidae or Barn Owls this operculum., as may be seen in 111. 48, E, 

 extends both above and below the aj^erture, which is roughly 

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