MORPHOLOGY OF THE OWLS. 268 



certainly does not contirm these beliefs — at least so far as the soft parts are concerned ; 

 which differed from those of S. ahtco only in that the vertical axis of the aperture was 

 slightly greater than the longitudinal axis of the eye, and that there was no asymmetry, 

 the size of the aperture being the same on each side of the head. 



Strix Jlammea concludes the list of those which 1 have been enabled to examine. The, 

 aperture of the ear is almost square ; its vertical axis is equal to half the longitudinal 

 axis of the eyelid. The preaural fold constitutes an operculum. 



The cavernum is not divisible into anterior and posterior divisions, but forms a deep 

 cavity leading almost directly into the caverntdum. 



The operculum, arising a short distance behind the gape as a low ridge, rapidly expands 

 into a square flap terminating some distance above the free edge of the aperture on a level 

 with the top of the eye. The base of the operculum measures about twice the length of 

 the aperture of the ear. 



The loose membranous postaural fold supporting the peripheral disc-feathers of Asio 

 otus is here represented by a low dermal ridge standing up distinctly from the surface 

 of the head. It arises on the inferior border of the lower jaw, and running backwards, 

 upwards, and forwards, terminates at the base of the beak. This ridge, as previously 

 intimated, serves to support the peripheral disc-feathers. These are much elongated, and 

 narrow-vaned, standing out at right angles to the head, and curved so as to present 

 a deeply concave surface forwards, and a corresponding convex sru'face looking backwards. 

 From their shaj)e and disjjosition these feathers afford an admirable apparatus for the 

 collection of sound. 



It is not surprising to find that the operculum varies slightly in form. In one specimen 

 (of two examined) the lateral border was shorter than that described above ; and in 

 consequence the superior border slopes gently upwards, giving the operculum a truncated- 

 conical rather tlian a square outline. 



It would seem, from these facts, that the form of the external ear was at some time 

 subject to considerable variations, the most successful of which have become more or less 

 fixed by selection. 



Results. 



That a detailed study of the Pterylography of the Owls would lead to any very startling 

 results was hardly to be expected, and, as the " Keys " at the end of this paper show, it 

 does little more than confirm wliat has alreadv been done. It must not be forErotten, 

 however, that these results have been obtained by a deliberate desertion of the beaten 

 track into the neglected highways and byways opened up by Nitzsch, but sinc<^ allowed 

 to sink almost into oblivion. 



I have been enabled to push Nitzsch's methods somewhat further than lie himself 

 attempted, for it will be remembered that he founded his genera rather upon the data 

 obtainable by the time-honoured methods of measurements and the relative proportions 

 in the length of the remiges, their number and general outline, and so on, rather than 

 upon pterylological characters, which seemed to him to differ little, if at all, from his 

 type Bubo. 



SECOND SERIES. — ZOOLOGY, VOL. VII. 3(1 



