MOEPHOLO&T or THE OWLS. 261 



specimen soon after death, gives the measurements of the external aperture as i~2 inch, 

 and describes it as of an elongated shape. " The ears," he writes, " are very low in the 

 head, in fact placed beneath the eyes, and appear, from external examination, to be quite 

 symmetrical ; but after the bird was skinned I could see, by looking at the skull from 

 the back, that the hind angle of the inferior mandible was lower on one side than the 

 other." 



Sijrnia ulula has a nearly circular apertnx'e, the vertical axis of wliich is equal to 

 the horizontal axis of the eyelid. The cavernum is spacious, divisible into anterior 

 and posterior portions, sharply defined ; the latter very deep, and passing insensibly into 

 the cavernulum. There is a considerable upward extension of the cavernum between 

 the skin and skull as far as the crown of the head. 



So far we have been discussing forms of the external ear which are perfectly 

 symmeti"ical on the two sides of the head. In the forms presently to be described 

 there is a marked asymmetry, with exceptions, however, which will be noticed in their 

 place. 



The first and most remarkable of these is that of Nyctala Tencjmalmi. The mem- 

 branous aperture is oval in form ; its vertical axis is twice the length of the longitudinal 

 axis of the eyelid. The preaural fold — the loose flap of membrane formino- the 

 anterior border of the apertm-e — forms an imperfect operculum (PL 27. figs. 5-6). 

 The cavernum, as in the forms just described, is divisible into an anterior and a posterior 

 portion, the latter being in this case by far the larger. By a considerable devcloijment 

 of the lateral wing of the squamosal and exoccipital bones, the form of the meatus 

 has been considerably modified, and is, as just hinted, asymmetrical on the two sides of 

 the head. 



On the left side this wing takes the form of a thin bony plate jutting out from the wall 

 of the cranium from behind, outwards, downwards, and forwards to the level of the post- 

 orbital process, so as to form a complete bony outer wall to the posterior portion of the 

 cavernum. It is continued downwards and forwards to overlap the articulation of the 

 lower jaw. 



The bony floor of the cavernum is incomplete, being hollowed out or " cut away " 

 to expose the quadrate from behind. Thus the posterior portion is converted into a 

 deep recess roofed over by bone, and looking outwards and forwards on the anterior 

 portion. 



On the right side the squamosal Ming does not extend along the cranium as far forwards 

 as the base of the postorbital process, but trends outwards and forwards to join the 

 outer border of its lower end, instead of being continued downwards to overhang the 

 articulation of the jaw as on the left side. 



Thus the posterior diAasion of the cavernum is converted into a deep recess, differing 

 from that of the left side in that it is open above and that it is shut oiT below from the 

 outer world and from a portion of itself by the junction of the squamosal wing with the 

 postorbital process. The portion cut off, just referred to, is represented by the deep 

 hollow over the superior border of the articular end of the jaw, and is well shown in 

 fig. 6, PL 27. 



