4 MK. W. P. PTCRAFT ON THE 



orbital processes stand out conspicuously from the skull, and extend downwards so as 

 nearly to touch the quadrato-jugal bar. 



Seen in section, the cranial roof presents some interesting features. As already stated, 

 this region of the skull is considerably thickened by the development of spongy tissue 

 which attains its maximum over the interorbital region. In Asio and JBiibo, for example, 

 the iab2da externa and tabula vitrea are of extreme thinness, and separated by a mass of 

 diploe to which has been added, in the interorbital region, a mass of pneumatic tissue. 

 Between the layei- of diploe, and, anteriorly, between the diploe and the pneumatic tissue, 

 there runs, from the parietal region forwards, a distinct pneumatic canal, which, 

 however, is lost in the region above the olfactory fossa. The great development of the 

 pneumatic tissue has caused the anterior end of the frontal to fold over upon itself so as 

 to overlap the base of the nasals and the proximal end of the nasal process of the 

 premaxilla. The pneumatic tissue, it is to be noticed, does not extend downwards 

 between the interorbital septum, save only along its anterior border where it runs 

 downwards and backwards to terminate at the great air-sinus, the anterior tympanic 

 recess. 



Syrnium differs conspicuously from Bubo in that the diploe and pneumatic tissue 

 alike present a regular cellular arrangement of superimposed layers — a series of 

 separate plates of bone supported by delicate bony pillars. This laminate arrangement, 

 however, terminates near the anterior end of the frontal, giving place to the normal 

 irregular spongy tissue. 



Strix agrees with Asio and Bubo in that the diploe and pneumatic tissue are of the 

 ordinary spongy type, having no regular arrangement ; but the skull differs both from 

 Asio and Syrnium in that the spongy mass descends downwards from the frontal to 

 inflate the whole of the interorbital septum. To such an extent has this development 

 proceeded, that the groove into which the olfactory nerve passes in its passage from the 

 skull along the roof of the orbit to the olfactory cavity has now become entirely 

 surrounded by the pneumatic tissue, the tabula exteima having been driven outwards, 

 leaving the groove, now converted into a tube, completely invested. In Eurystopus^ 

 one of the Caprimulgi, we have a stage halfway between the normal arrangement and 

 that in Strix, the nerve running in a deep trough of pneumatic tissue the edges of which 

 nearly meet to form a tube as in Strix. 



The Base of the Skull. — The basitemporal platform is roughly triangular in form, having 

 a very broad base, and its apex lying in the middle line between the basipterygoid 

 processes. Above the apex lie the Eustachian apertures. The anterior lateral borders 

 of the plate, save at the apex, have fused partly with the base of the parasphenoidal 

 rostrum and partly, on either side of the rostrum, with the wall of the recessits 

 tymjianicns anterior, so that the Eustachian grooves are converted into canals, which 

 open into the floor of the mouth of the recess. The paroccipital notch, which is deep in 

 many Ealconiformes, in the Striges is wanting, having been obliterated by the great 

 development of the meatus externus and the recessus tympanicus anterior. It is to this 

 same great development of the tympanic cavity that the loss of distinct paroccipital 

 processes is due, since the bony tissue used in their formation appears to have been 



