262 



1363. .The anchylosed parts of the cranium of a nearly full-grown Ostrich (Strut hio 

 Camelus). 



They consist of the basioccipital, basisphenoid and presphenoid, of the alisphenoids, orbito- 

 sphenoids and prefrontals, of the parietals, mastoids and petrosals. The posterior clinoid 

 processes overhang the back part of the deep sella, into the bottom of which the carotid 

 canals open. The foramina lacera anteriora form with the optic foramen a wide vacuity on 

 each side. The orbitosphenoids and prefrontals overarch these vacuities anteriorly and ex- 

 pand into a platform, supporting the fore part of the cerebral lobes and the small olfactory 

 ganglia, which are prolonged into the grooves continued from the anterior part of this plat- 

 form. A part of the sagittal suture remains. The thick, slightly sutural surface at the fore 

 part of the parietals and alisphenoids is exposed by the removal of the frontals. 



Presented by Prof. Owen, F.R.S. 



1364. The separated bones of the head of a young Ostrich (Struthio Camelus). 



Although, as a general rule in the Class Aves, the separate cranial bones can be discerned 

 only at a very early period, yet in those birds in which the power of flight is abrogated, the in- 

 dications of the primitive centres of ossification endure longer ; and in the species here selected 

 for the illustration of the cranial segments, the constituent, bones of the skull, with the excep- 

 tion of the basioccipital, i, and basisphenoid, 3, and the two bones, e and s, which coalesce 

 with the petrosal, ie, have been separated by maceration merely. 



The basioccipital (i) developes the major part of the single articular condyle, and sends 

 down a process, more marked in the Struthious genera, and especially the Dinornis, than in 

 most other birds : in all respects this primitively distinct bone retains the character of the 

 centrum of its vertebra. 



The exoccipitals, u, contributing somewhat more to the occipital condyle than in the Cro- 

 codile, develope, as in that reptile, the paroccipital (24) as an outstanding exogenous ridge or 

 process : but it is lower in position than in the Crocodile. The superoccipital (3), as com- 

 pared with that of the Crocodile (No. 763), manifests more strongly the flattening and deve- 

 lopment in breadth, by which the spinous elements lose the formal character from which their 

 name originated, and are converted from long into flat bones. It always protects the cere- 

 bellum ; is absent in the Frog where this organ is a mere rudiment ; and is present in the 

 Crocodile in the ratio of the superior size of the cerebellum. The further development of the 

 cerebellum is the condition of the superior breadth of the spine or crown of the epencephalic 

 arch in the bird. 



The second segment of the skull has for its central element a bone (a), the basisphenoid, 

 which in the bird, as in other ovipara, is connate with that (9) which stands in the same rela- 

 tion to the third cranial segment ; the proof of the natural distinction of these segments is 

 . given by the neural and haemal arches. Probably the circumstance of the bodies of those 

 vertebrae being formed by ossifications of the fibrous capsule of the notochord, representing 

 the external or cortical parts only of such centrums, may be the condition, or a favourable 

 physical cause of such connation. The neural arch of the parietal vertebra retains the same 



