106 REPORT — 1841. 



the superior laminae or neurapophyses, are all subject to such changes ; but 

 the neurapophyses are much more constant in their place of attachment than 

 the others. In Mammalia the ribs for the most part are joined to the inter- 

 space of two centrums ; in Reptiles each pair is attached to a single centrum. 

 In Fishes, and the Mosasaur among Reptiles, the hsemapophyses depend, each 

 pair from its proper centrum ; in other Reptiles and Mammalia they are arti- 

 culated to the interspace of two vertebrae, leaving a half-impression on each 

 of the contiguous centrums. The neurapophyses present a degree of con- 

 stancy in their relation to the body of the vertebra corresponding •with the 

 importance of their function. In Mammalia I know of no exception to the 

 rule, that each neural arch is supported by a single centrum : in Birds no ex- 

 ception has hitherto been recorded ; but among Reptiles the Cheloniae* oiFer 

 in those vertebrae, 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 superior laminae 

 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 ; each 

 neural arch is there supported by two contiguous vertebrae, the interspace 

 of which is opposite the middle of the base of the arch above, and the ner- 

 vous 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 distinguishing characters which have been recognized in the dorsal and 

 caudal vertebrae, in regard to the smooth and polished surface of their middle 

 constricted part ; the cylindrical, or nearly cylindrical transverse contour of 

 this part below the lateral depression ; their expanded, thickened and rounded 

 articular margins, and also, though in a somewhat less degree, their relative 

 length as compared with their breadth and height. The three middle sacrals 

 are, however, somewhat shorter than the two terminal ones. 



In. Lin. 



Antero-posterior diameter of centrum of fifth sacral ... 4 10 



Vertical diameter of centrum of fifth sacral 4? 1 



Transverse diameter of centrum of fifth sacral 4 6 



Vertical diameter of the middle of the body 2 6 



Total height of fourth sacral vertebra II 



The neural arches of the first three sacral vertebras rest directly over the 

 irrterspaces of the subjacent bodies ; that of the fourth derives a greater pro- 

 portion of its support from its proper centrum ; aud that of the fifth, which 

 rests by its anterior extremity on a small proportion of the fourth centrum, 

 is extended over nearly the whole length of its own centrum, so that in the 

 caudal vertebrae the ordinary relations of the neural arch and centrum are again 

 resumed. In the four first sacral vertebrae the base of the neural arch ex-' 

 tends half way down the interspace of the bodies, and immediately developes 

 from its outer part a strong and short transverse process (broken and rounded 

 off in the fossil). From the base of this process the neurapophysis expands 



* Cuvier describes the exceptional structure above alluded to in these Reptiles, and like- 

 wise cites the Chondropterygians ; ' Le9ons d'Anat. Comparee,' ed. 1836, torn. i. p. 213. 



