124 SHUFELD T. [Vol. XV 1 1 . 



another type of cranial structure. In it the orbits are notably 

 large, and the mandibular part strong. The jugal bone is 

 linear, and the supermedial furrow of the cranium is well 

 marked. The auricular openings in most of their characters 

 agree with those forms already described, in being relatively 

 small or of medium size, and in lacking any external dermal 

 conch, or ear-flap. 



Group II. Aural skin-flaps absent. Cranium symmetrical. 

 Auricular apertures largest upon the right side. 



4. Bubo ignavus (Forst.). 



Ear-openings of medium size; nearly of the same dimensions. Osseous 

 crest of the os sqtiamosum comparatively small, completely free above ; 

 viewed anteriorly, they are almost concealed by the orbits. Posterior 

 periphery of either orbit sharp where formed by the frontal bone. Cranium 

 has a supermedial furrow present ; its greatest height is posterior to the 

 orbits. Jugal linear. Vomer present. Supraoccipital foramen present. 



To this, the second group, belongs Bubo ignavus, which is 

 further characterized by possessing a symmetrical cranium, and 

 the lack of dermal ear-flaps, but in this species the first evi- 

 dence of asymmetry exhibits itself in an insignificant differ- 

 ence in the size of the two ear-openings. Here the cranium 

 has a structure most like Nyctea scandiaca, and has, as in that 

 species, powerfully developed mandibles, a conspicuous median 

 furrow on the supero-external aspect of the cranium, broad 

 and prominently outstanding orbital wings, markedly capa- 

 cious orbits, and feebly developed osseous crests on either 

 OS squamosum. 



Owing to the feeble development of these processes we may 

 infer that the sense of hearing in this owl is comparatively 

 less acute than it is in any other North-European species. 

 The similarity in the structure of the cranium in these two 

 species is, upon the whole, so close that doubtless they would 

 have both been relegated to the same group had not the 

 right ear-opening in B. ignavus been a little larger than the 

 left, a state of affairs that I have not been able to demonstrate 

 with certainty in the case of Nyctea. 



