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, 1, 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 (2s) and the pterygoid (21) 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 newrapophysial 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 hemal 
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 vertebre 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 hemal arch of the occipital 
vertebra has retained much of its embryonic proportions. The pleurapo- 
