OSSEOUS SYSTEM OF AVES. 63 



ward, with extreme tliinning and sometimes partial loss of bone. 

 The roof of the orbit is formed by the frontal, prefrontal, and 

 lacrymal ; the hind wall by the frontal, ali- and orbito-sphenoids ; 

 there is no bony floor ; but the eyeball rotates on a sort of air- 

 cushion resting upon the palatal, the pterygoid, and the orbital 

 process of the tympanic. The bony septum is usually more or 

 less incomplete, and the orbital freely communicates with the 

 temporal vacuity. Only in a few species is the periphery of the 

 orbit completed by bone, as in certain Maccaws and Cockatoos 

 (^Macrocercus, fig. 30, Plyctolophus, Calyptorhynchus) \ the la- 

 crymal extending to the postfrontal as a continuous suborbital 

 bar. In the Woodcock the large lacrymal so extends the front 

 wall of the orbit as to cause it to look a little backward as well as 

 outward : and the orbits are so large as to push the brain-case to 

 the lower and back part of the cranium. In the Owls the post- 

 frontals have the form of broad thin plates, compressed from 

 before backward, and unusually produced downward to increase 

 the wall of the large orbit and give it a more anterior aspect. In 

 most diurnal Raptores the upper wall of the orbit is supple- 

 mented by a dermal oblong flat superorbital bone, ligamentously 

 connected with the lacrymal. The orbits are smallest and worst 

 defined in the nocturnal small-eyed Apteryx : there are no super- 

 orbital ridges, no antorbital or postorbital processes, and the inter- 

 orbital septum is complete and thick, the optic foramina being 

 wide apart. In Dinornis the orbits are small, and also divided 

 by the rhinal chamber : but the superorbital ridge is present and 

 developes a strong postorbital process. The interorbital septum, 

 as a rule, is very thin, even when entire, as in T achy petes, 

 Coracias, Eury stomas \ it may have a small vacuity (^Aquilci) or 

 a very large one {Buceros^, or two or three as in most birds. 



The olfactory cerebral crura emerge from the cranium at the 

 upper angle between the hind wall, roof, and septum of the orbit ; 

 groove the upper part of the septum as they pass forward to 

 penetrate the prefrontal and expand into the rhinencephalon, 

 dispersing the olfactory nerves to the turbinal membranes. The 

 frontal olfactory foramen, in the Raptores, is smaller than the 

 prefrontal one. Between the Vulture and the Crocodile the dif- 

 ference is that the rhinencephalic crura extend along a common 

 canal above the interorbital space in the Reptile, while in the 

 Bird the ossification of the septum divides the rhinencephalic 

 fossa into two : but many birds resemble the Crocodile in this 

 respect. The bones which hold the neurapophysial relation to 

 the rhinencephala, anterior to the frontals, are the same, or homo- 



