No. I.] THE CRANIUM IN THE OWLS. 125 



Group III. Ear-flaps present. Cranium symmetrical. Ear- 

 openings asymmetrical, resembling a gill-slit, but the aural 

 skin-flaps of equal size. 



5. Asio AcciPiTRiNus (Pall.). 6. Asio otus (Linn.). 



Auricular fissures or slits carried high up. The entrance to the ear is 

 on the right side below, and on the left side above an outstretched fold 

 of the skin. Osseous crest of the os squafnosiim, upon either side, meets 

 with the frontal bone above, without any intervening notch. Viewed from 

 in front they are wholly visible beyond the orbits. Frontal bones obliquely 

 sculptured at the posterior margins of the orbits ; the sculpturing being 

 most marked in A. ottcs. External supermedial furrow of the cranium 

 present ; greatest vertical depth of cranium lies in the postero-orbital plane. 

 Jugal linear. Vomer present. Supraoccipital foramen generally found. 



The third group contains the two species of Asio, vis. : 

 A. accipitrinus and A. otus. In the case of these also is the 

 cranium symmetrical, while the auricular openings and their 

 dermal appendages exhibit in their structure a very remarkable 

 asymmetry. The apertures to the ears are so markedly wide 

 that they remind one of gill-slits, inasmuch as they extend 

 from the nether side of the mandible, upon either side, up to 

 a point near the middle of the forehead, where they are sepa- 

 rated only by a small interval of space. Upon either side of 

 this long slit-like aperture there is a raised fold of skin, the 

 two resembling a pair of lids, of which the supero-anterior one 

 is the real ear-flap. Crossing the aperture is an elevated fold 

 of skin, and it is in its neighborhood that the asymmetry is 

 especially observable. To its left side, and above it, we find 

 the entrance to the ear, while on the right side of the head 

 it lies below this fold. So far as the cranium itself is con- 

 cerned it does not exhibit any special asymmetry. The 

 osseous crests of the squamosal bones are peculiar in form, 

 inasmuch as they ascend up to the frontal, upon either side, 

 without any intervening notch, whereby they are endowed 

 with an unusual amount of superficial surface, and with great 

 depth ; this, taken in connection with the unusual development 

 of the external ear-openings, accounts for the sense of hearing 

 having attained to its greatest acuteness in this group. The 



