"92 BULLETIN UNITED STATES GEOLOGICAL SUEVEY. fFo^vI. 



'They are rather thin, concave outwards, very slightly movable at their 

 opposed edges, and carry out their usual function of maintaining the 

 form of the optical apparatus. The orbital periphery very nearly ful- 

 fills that rare condition in birds of a complete bony circuit. It is 

 only deficient at the point where the lachrymal fails to touch the 

 maxillary. The upper mandible of this bird is made the more conspicu- 

 ous and distinct from the remainder of the skull by the abrupt way in 

 which it is attached and the miich firmer texture of the bone. The 

 mandibular culmen is perfectly convex from the tip of the sharp-pointed 

 extremity to where it suddenly terminates under the slightly overhang- 

 ing frontals, or, more correctly, the minute surface appearance of the 

 l^refontal, for although it is not evident in the adult that that bone 

 makes itself visible at this point, yet it may be demonstrated in skulls 

 of younger specimens. The culmen, as in other birds, is formed by the 

 intermaxillary, which is here firmly united with the nasals, and the two 

 in conjunction form the peripheries of the truly elliptical external nasal 

 apertures or nostrils, the first bone bounding them anteriorly, while the 

 latter completes their arcs in the rear. These in the dry skull measure 

 through their major axes seven millimetres, and through their minor 

 ones barely five millimetres. They have a distinct ring raised around 

 their circumference, which is wanting, however, where they nearest ap- 

 proach each other anteriorly at the culmen. The plane of the nostril faces 

 upwards, outwards, and forwards ; the nostrils are completely separated 

 from one another by a vertical bony septum, developed from the intermax- 

 illary, not a common occurrence in birds. They have, in addition, a con- 

 cave bony floor, that rises behind into a posterior wall, leaving really two 

 semicircular openings just beneath the culmen, separated from each other 

 by the vertical septum. The osseous mandibular tomium, also a part of 

 the intermaxillary, is as sharp as when the bill is sheathed in its horny 

 integument. The arc is concave, and falls off rapidly as it api)roache8 

 the tip of the beak. Occasionally, in very old birds, the ethmo-turbinal 

 bones in the nasal passages may ossify. The nasals form here the sides 

 of the bill, and are firmly auchylosed to the bones they meet, except the 

 lachrymals. The movability of the fronto-mandibular articulation is 

 limited. The dry skull is extremely light and brittle, giving one the 

 sensation in handling it that he might experience while examining an 

 egg from which the contents had been removed. A line drawn from the. 

 tip of the upper mandible to the outermost point of one tympanic, 

 around the arc of the cranium to a similar point on the opi^osite side, 

 and back to the point of departure, describes nearly the sector of a circle. 

 The longest radius, which is in the median line, measures four and one- 

 half centimetres, the chord between the tympanies about three centi- 

 metres. 



The hyoid arch. — The hyoid arch is suspended from the base of the 

 skull by its usual attachments. In this Owl it consists of but six very 

 delicate little bones, involving five articulations. The tips of the up- 



