OSSEOUS SYSTEM OF AYES. 49 



nium of Dinornis retain much of their crocodilian position, but 

 they hold a lower one in the loftier domed crania of other birds : 

 they vary in the developement of their apophysial part, standing 

 further out, e.g., in Rhea and Struthio, than in Dromaius. The 

 occipital region is bounded above by the arched ridge formed by 

 the insertion of the muscles longus colli posticus and complexus, 

 in large and powerful birds ; and is bisected, as, e. g., in the Eagle, 

 by a median-vertical ridge dividing the transverse one into a pair 

 of arches : in Dinornis a prominence between the insertions of 

 the longus colli jjosticus and complexus subdivides the transverse 

 ridge into four arches; and, here, a lower transverse ridge, 

 bounding the insertions of the ?^ecti cap. postici and traclielo- 

 mastoidei, overarches the foramen magnum. ^ In smaller and less 

 robust birds a cerebellar prominence marks the middle of the 

 occipital region : in some species the pressure of the brain from 

 within, and the muscles from without, reduces the thin, bony 

 wall in some places to its membranous lining, leaving openings in 

 the dry skull commonly on each side of the cerebellar prominence. 

 These have been termed ' fontanelles,' as if they were due to 

 original arrest of cranial ossification, but the latter explanation 

 applies to openings, usually reduced to a venous outlet, between 

 the exoccipital and mastoid. In certain Doves, Owls, Parrots, 

 and the Dodo, there is a median ^ superoccipital ' foramen, usually 

 accompanied by a pair of venous foramina. 



The basisphenoid chieily differs in the presence or absence of 

 ^ pterapophyses.' ^ They are longest in the StruthionidcB, fig. 27,j,^ 

 are short and thick in Dinornis and Apteryx, and abut against the 

 tympanic end of the pterygoids ; they are shorter in Grallatores 

 ( Vanellus) and Rasores ( Columha, Tinamus, Syrrhaptes), and 

 their abutment is nearer the middle of the pterygoids ; they are 

 absent, or are too short to reach the pterygoids, in the Dodo, 

 Owls, Diurnal Kaptores, and most other Birds. In the Emeu, 

 Apteryx, and Dinornis, the basisphenoid shows a median perfora- 

 tion. The sides of the basisphenoid, obliquely grooved by the 

 Eustachian canals {Dinornis) and excavated to form the base of 

 the tympanic cavity, in some birds extend outward to the tym- 

 panic process of the mastoid, and with it grasp the hinder condyle 

 of the tympanic. 



The mesencephalic fossa and the ' foramen ovale ' for the transit 

 of the fifth or trigeminal nerve indicate the alisphenoid, fig. 28, 6, 



* xvr. vol. iii. pi. 52, fig. 4. 



"^ First indicated as such in xvi*. vol. iii. p. 351 (January, 1848); see also xliv. p. 



303, no. 1601. » xliv. p. 259. 

 VOL. II. E 



