294 REPORT — 1846. 



tions of the cephalic division of the vertebrate endoskeleton. Although, as a 

 general rule, 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 indi- 

 cations of the primitive centres of ossification endure longer, and in the 

 species here selected for the illustration of the cranial segments (fig. 23) the 

 constituent bones of the skull, though figured of their natural size, have, with 

 the exception of the basioccipital, i, and basisphenoid, 3, and the two bones, 

 6 and s, which coalesce with the petrosal, 16, been separated by maceration 

 merely. I may remark, however, that in all birds, certain bones, which 

 coalesce with others in the cranium of most mammals, always retain their 

 primitive individuality ; the tympanic (29) and the pterygoid (24) for ex- 

 ample. 



The hindmost segment of the cranium (N 1, fig. 23) so closely repeats the 

 characters of the epencephalic neural arch of the crocodile (fig. 18), as to 

 render a separate and full view of it unnecessary for the illustration of its 

 vertebral character. The basioccipital (1) still 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, 12, contributing somewhat more to the occipital condyle 

 than in the crocodile, 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 proper neurapophysial characters of no. 2 are fully main- 

 tained. The supraoccipital (3) now begins to manifest more strongly the 

 flattening and development in breadth, by which the spinous elements lose 

 the formal character from which their name originated, and are converted 

 from long into flat bones. We saw the first step in this most common of the 

 changes to which one and the same endoskeletal element is subject, in the 

 detached neural spine of the atlas of the crocodile : that of the occipital 

 vertebra of the same animal presented another stage in the metamorphosis: 

 we have a third degree in the bird, and the extreme of expansion is attained 

 in the human subject (fig. 25, 3), where the spine is sometimes developed, 

 like that of the parietal vertebra, from two centres. But the arrested steps 

 in this strange change of form and proportion demonstrate the essential 

 nature of the part, as the neural arch, whilst the constancy of the characters 

 of connexion is shown by this crown of the arch of the occipital vertebra 

 having the exoccipitals as its piers or haunches from the fish to the human 

 subject. It always protects the cerebellum ; 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 cere- 

 bellum is the condition of the superior breadth of the spine or crown of 

 the epencephalic arch in the bird. 



The arguments that determined the nature and displacement of the haemal 

 arch of the occipital vertebra in the crocodile apply with equal force to that 

 in the bird. The extent of the displacement, it is true, has been greater ; 

 not seven, but seven-and-twenty vertebrae may intervene between the place 

 of the scapulo-coracoid arch and the remainder of its proper segment con- 

 stituting the occipital region of the simple cranial box in the bird. But this 

 difference of extent ought no more to mask the real relationship of such 

 costal arch to its centrum, than the degree of development of the spine of 

 the occipital vertebra affects the general homology of that element. 



In the ostrich, and other struthious birds, the haemal arch of the occipital 

 vertebra has retained much of its embryonic proportions. The pleurapo- 



