WEALDEN FORMATIONS. 9 



other elements. In Mammals the ribs for the most part are joined to the interspaee 

 of two centrums ; in reptiles each pair is attached to a single centrum. In fishes, and 

 the Mosasaur among reptiles, the haemapophyses depend, each pair from its proper 

 centrum : in most other reptiles and mammals they are articulated to the interspace of 

 two vertebrae, leaving a half-impression on each of the contiguous centrums. The 

 neurapophyses present a degree of constancy in their relation to the body of the 

 vertebra corresponding with the importance of their function. In Mammals I know of 

 no exception to the rule, that each neural arch is supported by a single centrum : 

 among reptiles the Chelonia* offer in those vertebras in which the expanded spinous 

 processes contribute to form the carapace, the interesting modification analogous to 

 those noticed in the lateral and inferior vertebral elements, viz., a shifting of the neura- 

 pophyses from the middle of the body to the interspace of two adjoining centrums, 

 whereby that part of the spine subject to greatest pressure is more securely locked 

 together, and a slight yielding or elastic property is superadded to the support of the 

 neural arch. 



The same modification is introduced into the long sacrum of birds ;f each neural 

 arch is there supported by two contiguous vertebras, the interspace of which is opposite 

 the middle of the base of the arch above, and the nervous foramen is opposite the 

 middle of each centrum. It is this structure, beautifully exemplified in the sacrum 

 of the young ostrich, which Creative Wisdom adopted to give due strength to the 

 corresponding region of the spine of a gigantic Saurian species, whose mission in this 

 planet had ended probably before that of the ostrich had begun. 



The anchylosed bodies of the sacral vertebrae of the Megalosaur retain the distin- 

 guishing characters which have been recognised in the dorsal and caudal vertebrae, in 

 regard to the smooth and polished surface of their middle constricted part ; the cylin- 

 drical, or nearly cylindrical transverse contour of this part below the lateral depres- 

 sion, e; their expanded, thickened, and rounded articular margins, i; and also, though 

 in a somewhat less degree, their relative length as compared with their breadth and 

 height. The three anterior sacrals, T. I, 1,2,3, are, however, somewhat shorter 

 than the two posterior ones, 4 and 5. 



The following are the dimensions of the fifth sacral vertebra: 



Antero-posterior diameter of centrum 

 Vertical diameter of articular and of centrum 

 Transverse diameter of the same part 

 Vertical diameter of the middle of the centrum 

 Total height of the fourth sacral vertebra . 



* Cuvier describes the exceptional structure above alluded to in these reptiles, and likewise cites the 

 Choudropterygians, 'Lemons d'Anat. Comparee,' ed. 1836, torn, i, p. 213. 

 f 'On the Nature of Limbs/ 8vo, 1849, p. 61, fig. 10. 



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