298 REPORT—1846. 
developed than is the front one of the centrum of the atlas, with which, in- 
deed, it is loosely connected by ligament. The expanse of the occipital 
‘spine, 3, has been governed, agreeably with a foregoing remark, by the su- 
perior development of the cerebellum. : 
The hzemal arch of the occipital vertebra is represented, like those of the 
cervical vertebra, by the plewrapophysial elements only (51); but these, in 
most mammals, are developed into broad triangular plates with outstanding 
processes: that called ‘spine’ and ‘acromion’ is exogenous ; but that called 
‘coracoid’ is always developed from an independent osseous centre (a rudi- 
mental representative of the hemapophysis, 52), which coalesces with the 
pleurapophysis in mammalia, and only attains its normal proportions, com- 
pleting the arch with the hemal spine (episternum) in the monotremes. 
In many mammals the arch is completed by bones, which are, apparently, 
the hemapophyses of the atlas, e. g. in man (fig. 25, 52’), which have followed 
the occipital heemal arch in its backward displacement, but not quite to the 
same extent. 
The diverging appendage, though retaining the general features of its 
primitive radiated form, has been the seat of great development and much 
modification and adjustment of its different subdivisions (53-57) in relation 
to the locomotive office it is now called upon to perform. 
With the exception of this excess of development of the appendage, the 
defective development and displacement of the hzmal arch, and the coales- 
cence of the parapophyses in the neural arch, there are few points of resem- 
blance which are not sufficiently salient between the segment N1, H1 in the 
mammal, and that so marked in the fish (fig. 5). And, if the interpretation 
of the more normal condition of this segment in the lower vertebrate, ac- 
cording to the archetypal vertebra, fig. 15, be accepted, then the explana- 
tion of the nature of the modifications of the special homologues of the con- 
stituents of the occipital segment by which that archetype is masked in the 
mammal, may be confidently left to the judgement of the unbiassed student 
of homological anatomy. 
In commencing his comparisons of the second segment of the skull with the 
typical vertebra, he will be unexpectedly gratified by finding, in the immature 
mammal, the centrum, 5, naturally distinct, and the hemal arch, H.1, retaining 
its natural connections with the rest of the segment, and by means of a more 
complete development of the pleurapophyses (3s) than in any of the inferior 
air-breathing vertebrates. He may now separate, without artificial division of 
any compound bone, the entire parietal segment, but he brings away with it 
the petrified capsule of the acoustic organ, and the anchylosed distal piece (27) 
of the maxillary appendage, which more or less encumbers and conceals the 
typical character of the neural arch of the parietal vertebra in every mammal : 
least so, however, in the monotremes and ruminants. The newrapophyses (6) 
of the parietal vertebra, like the mesencephalic segment of the brain, are but 
little more developed in mammals than in the cold-blooded classes: they are 
notched in the hog and perforated in the sheep by the larger divisions of 
the trigeminal, and they send down an exogenous process, which articulates 
and sometimes coalesces with the appendage (24) of the palato-maxillary 
arch. The neural spine (7), always developed from two centres, often vastly 
expanded, and sometimes complicated with a third interealary or inter- 
parietal osseous piece, is occasionally uplifted and removed from its neur- 
apophyses by the interposed squamous expansion of the bone 27; but this, 
which reminds one of the occasional separation of the neural arch from the 
centrum of the atlas in fishes, is a rare modification in the mammalian class. 
A still rarer one is the separation of the halves of the parieto-neural spine 
