262 ME. W. P. PTCEAFT ON THE 



In Asia the asymmetry does not extend to the bones of the skull, hut is confined to 

 certain membranes uithin the cavernum. 



In A. accipitrinus (PI. 27. figs. 3— i) the membranous aperture is crescentic in form and 

 of great size, extending from the middle of the lower jaw, from a point corresponding 

 to a line drawn from the base of the cere downwards, backwards, and upw^ards to a point 

 over the region of the middle of the eye. Thus the cavernum has gained an enormous 

 superficial area, such as has been liinted at in other Owls, in the extension of the 

 cavernum over the skull, and backAvards round to tlie occiput. The pre- and postaural 

 folds are very A'olumincnis, the former being kno\\Ti as the opercidum. 



That portion of the cavernum which lodges the eye, and the cavity behind, corresponds 

 to the anterior and posterior divisions of the cavernum in other species. The latter 

 division is here much increased, and, by reason of the disposition of certain membranes, 

 has come to be asymmetrical as regards the two sides of the head. 



On the right side this will be seen to be divided into an upper and a lower chamber by 

 means of a membranous horizontal partition running from the squamosal wing to the 

 eye, and forward to the operculum. The upper chamber is blind, and may therefore be 

 called the diverticulum. It is possible that it may act as a sort of resonator. The lower, 

 which should, I would suggest, be called the via recta, is very spacious and leads to the 

 caverntdum. It is bounded above by the floor of the diverticulum; behind by the 

 squamosal wing ; below by a thin membrane from the postaural fold to the operculum, 

 running immediately above the superior border of the lower jaw ; and in front by a thin 

 membrane from the lower jaw to a bony tul^ercle on the postorbital process. 



On the left the diverticulum lies below the via recta. The membrane dividing the 

 two is placed nearer the lower jaw, and sloj^es oljliquely downwards and inwards. It is 

 also partly attached to the tubercle of the postorbital process just referred to — which 

 tubercle, by the way, occurs o)ily in this species. 



The ear of ^. otus differs from the species just described in that there is no postorbital 

 tubercle, that the via recta of the left side is without the anterior boundary-membrane, 

 and in that the diverticulum of the cavernum is narrow and deep instead of broad and 

 shallow. 



Syridmn furnishes iis with a third type of modification of the external ear. 



In .S. aluco it is asymmetrical, but the asjTnmetry is confined to the membranous 

 aperture, which is semilunar in form. The vertical axis of the aperture of the right side 

 is equal to the horizontal axis of the eyelid ; that of the left aperture is less. 



The cavernum is divisible into anterior and posterior portions, the former larger and 

 shallow'er, the latter deep, but confined to the lower half of the posterior corner of the 

 cavernum, the floor of which is fairly spacious. The upward continuation of the 

 cavernum over the skull is but slight. The pre- and postaural folds are large, the 

 former, constituting the operculum, overlapping the free edge of the postaiu-al fold wdien 

 the ear is closed. 



S. uralense is generally believed to be asymmetrical as regards the form of the 

 external ear, and, moreover, the asymmetry is further supposed to extend to the bones of 

 the skull. A specimen in the flesh from my late friend Mr. Meinertzhagen's aviaries 



