18/1.] MR. COLLETT ON THE SKULL OF STRIX TENGMALMI. 741 



tinues without any incision of its margin down to the ' os occipitale.' 

 From this cause there exists, in the two species last named, to a 

 greater degree than in the others, a wall, open in front and closed 

 behind, projecting to the sides. When the skulls are seen in front 

 the ' cristse ' appear in these two species very distinctly, while in the 

 rest they are almost completely hidden behind the 'processus orbi- 

 talis posterior.' I have not had an opportunity of examining the 

 other species of the northern fauna not named here (Strix funerea, 

 S. liturata, S. lapponica and S.jisilodactgla) ; but I have seen several 

 non-European species. 



The wall just mentioned is peculiarly adapted to throw back the 

 vibrations of sound and carry them on to the * membrana tympani ' 

 in all the species which I have examined. I am therefore inclined 

 to attribute to the nearly allied Strix otus and S. brachyotus a 

 highly developed power of hearing. In a still greater degree, how- 

 ever, is this the case in S. tengmalmi, where this bone, and by conse- 

 quence several of those adjoining it, has a very peculiar formation. 



In Strix tengmalmi the ' crista' which the 'os squamosum ' pushes 

 forth in front of the ear-opening projects more considerably than in 

 the other species ; and as the outer edge of the ' crista ' widens about 

 the middle into a long tongue-shaped process, the whole ' os squa- 

 mosum ' surrounds a larger and deeper ear-cavity than in any of 

 the other species with which I am acquainted. 



But at the same time a very remarkable asymmetry appears in this 

 'crista.' On the right side (see figs. 2, 3, & G, p. 740) it projects 

 far upward and behind the upper edge of the ' os alisphenoideum ;' 

 and on the left side (see figs. 2, 4, & 6, p. 740), on the contrary, 

 it leans immediately on the foremost edge of that bone and some- 

 what beneath its upper part. On both sides it normally joins in its 

 lower part the ' os occipitale laterale ' extending on the right con- 

 siderably more (its height being here 20 mm.) than on the left, 

 where its height is only 15 mm. On the right side, where this ' crista' 

 lias its largest extent and at the same time is drawn furthest back, it 

 apparently projects at the top behind the point where the projecting 

 end of the ' os parietale ' meets the upper edge of the ' os squa- 

 mosum.' Between where the 'crista' begins and the foremost edge 

 of the remaining part of the ' os squamosum ' (that point at which 

 the last-mentioned bone touches the hinder edge of the 'os alisphe- 

 noideum') there is a proportionately considerable distance, amounting 

 in four of the skulls examined by me to 7'25 mm. On the left side, 

 on the contrary, the 'crista' projects at the top immediately at the 

 point which the ' os squamosum ' forms with the ' parietale,' ' fron- 

 tale,' and the hinder edge of the ' alisphenoideum.' Here the ' crista ' 

 leans upon the upper (but not the uppermost) end of the ' processus 

 orbitalis posterior.' Though this ' crista ' at its upper end joins the 

 ' os squamosum ' very differently on either side, its lower part is 

 apparently symmetrical and proceeds normally on both sides to the 

 lateral parts of the perfectly symmetrical ' occipitale.' 



In most of the Northern Strigidce the outer edge of the ' crista ' 

 has a distinct incision, bending downwards to the ' os occipitale ' 



