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 haemal arch of the occipital vertebra is represented, like those of the 

 cervical vertebra, by the pletirapophysial 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 hcemapophysis, 52), which coalesces with the 

 pleurapophysis in mammalia, and only attains its normal proportions, com- 

 pleting the arch with the haemal spine (episternum) in the monotremes. 



In many mammals the arch is completed by bones, which are, apparently, 

 the hagmapophyses of the atlas, e.g. in man (fig. 25, 52'), which have followed 

 the occipital haemal 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 haemal 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 Ni, H 1 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 haemal arch, H rr, 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 nearapophyses (e) 

 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 intercalary 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 



