OSTEOLOGY OF BIRDS 



55 



At the back of the orbit the wall is broad and gently concave 

 throughout ; it being pierced at its lower and inner angle by a 

 circular optic foramen, and the foramina more external to it are 

 quite distinct from each other, which is by no means the rule gen- 

 erally among birds. 



The outline of the olfactory foramen leading into the brain case 

 is very irregular, and the wall in its immediate neighborhood is 

 thinned to the extent of perforation in one specimen before me, 

 while in another two minute foramina occur, just large enough, on 

 either side, to admit the passage of the nerves, and the aforesaid 

 perforation is much smaller. Quite an extensive osseous flap is 

 thrown out to shield the opening to the ear behind. This latter 

 aperture is comparatively very large, the opening being fully equal 

 in size to the corresponding one in a specimen of Falco r. 

 g y r f a 1 c o from Alaska, which I find in my collection, and, as we 

 know, a very much larger species than Circus. In the upper part 

 of the recess, formed by this aural cavity, the double head of the 

 quadrate articulates, the outer head with the squamosal, the inner 

 one with the bony wall within. This bone then becomes twisted 

 on itself, to support below the usual articular facet for the mandible, 

 which facet is quite narrow from before backward, and rather 

 long transversely. It presents two articular surfaces, an outer and 

 an inner, connected by a narrow isthmus posteriorly, and separated 

 by a shallow concavity anteriorly. 



The quadrate throws inward a stumpy orbital process, the an- 

 terior surface of which lies in the same plane with the general an- 

 terior surface of the bone, it being directed upward, forward and 

 outward. On the posterior surface of the quadrate we find a 

 longitudinal depression coming down from between- the two heads 

 mentioned above, which harbors one of the pneumatic foramina, the 

 other being found at the base of the orbital process on this aspect. 

 The peculiar form of the cranial vault with the bulging supra- 

 occipital prominence, should be noted on this lateral aspect of the 

 skull. 



Upon a superior view of the skull of Circus [fig. 20], the 

 principal points to be observed are the position of the elliptical, 

 osseous nares ; the direction of the craniofacial suture, which in this 

 harrier is not drawn directly across in a transverse line, as it is in 

 Falco sparverius, for instance. It is to be observed also 

 that the sutural traces of the nasal processes of the premaxillary are 

 quite distinct in adult skulls, while in some falcons they are entirely 



