48 ANATOMY OF VERTEBRATES. 



segments of the endoskeleton in the bird's skull are the petrosal, 

 16, between the neural arches of the occipital and parietal verte- 

 brae, connate or co-ossifying with the elements of those vertebrae 

 with which it comes in contact ; the sclerotals, 1 7, interposed be- 

 tween the frontal and nasal neural arches ; and the thyrohyals, 

 47, retained in connection with the debris of the haemal arch of the 

 parietal vertebra. The olfactory capsule may be represented by 

 ethmoturbinal and turbinal processes in the skull ; but chiefly 

 remains cartilaginous. The dermal bone (lacrymal), 73, is Avell 

 developed and constant : one or more superorbital dermal bones 

 are occasionally present. 



As the characters of the occipital segment of the bird's cranium 

 are so obviously those of the vertebral neural arch as to compel 

 acceptance of the interpretation of its elements according to the 

 terms of general homology, there is a priori probability in the 

 segmental type being continued to the front end, as it is to the 

 hind end, of the vertebral axis, notwithstanding the modifying 

 influences of the large intercalated sense-capsules, and of the 

 special uses to which certain of the lower bony arches are des- 

 tined in the head. Developement, obedient to these demands, 

 gives such evidence as it can in favour of the presumption, while 

 relative position and connections afford the proof. In the fore- 

 going description I have, therefore, explained the chief constitu- 

 tion of the bird's skull in the terms of general homology, and I 

 proceed to point out some of its principal modifications in those of 

 special homology. 



The occipital condyle is single in Birds, varying from the 

 hemispheroid to the transversely elliptic form, with sometimes a 

 median notch or pit, and in the extent to which it projects; being 

 pedunculate in Dinornis,^ but sessile as a rule. The foramen 

 mao-num varies from a subcircular to a full transverse ellipse, 

 and to a vertically oval {Di?ior?iis giganteus) form, ^\i\\\ lateral 

 encroachments as in Aptomis"^ and Didus;^ its plane may be 

 vertical (ib.), but as a rule is oblique from above downward and 

 forAvard, thus departing further from the reptilian character. 

 The basioccipital in the extinct Aptornis sends down, as in the 

 Crocodile, a deep plate below the condyle : it descends in a less 

 degree in Dinornis, in both swelling out laterally into a pair of 

 tuberosities, completed by ossifications in the basisphenoid carti- 

 lage which afterwards coalesce.'' As a rule the condyle is on a 

 level with the basis cranii. The paroccipitals in the low flat cra- 



• XVI-. vol. iv. pi. 24, fig 2. ^ lb. vol. iii. p. ?50. 



2 lb. vol. iii. pi. .52, fig. 4. ^ lb. fig. 1, 5'. 



