338 BRITISH FOSSIL REPTILES. 



The neural arches of the first three sacral vertebrae, ns 1,2, 3, have been advanced 

 so as to rest directly over the interspaces of the subjacent bodies ; that of the 

 fourth, )is 4, derives a greater proportion of its support from its proper centrum, c 4 ; 

 and 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 first four sacral vertebrae the base of the neural arch extends half 

 way down the interspace between the bodies, and immediately developes from its outer 

 part a strong and short transverse process, or parapophysis, p, which is broken and 

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

 upward, forward, and backward, is joined by vertical suture to similar expansions of the 

 contiguous neurapophyses, and terminates above in a ridge of bone, < </, at right angles 

 to the suture ; this ridge, Avitli those of the other neurapophyses, extends longitudinally 

 above the parapophyses the whole length of the sacrum, and forms the margin of the 

 platform from which the spinous and accessory processes are developed : in the last 

 sacrum the corresponding part forms a thick, obtuse process, or diapophysis d, j. The 

 platform is further supported by a compressed ridge of bone extended from the upper 

 part of the parapophyses, like a buttress, to the middle of the horizontal ridge. On 

 each side of the buttress there is a depression, which is deepest in front. The 

 spinous process is not developed, as in the dorsal vertebrae, immediately from the 

 platform, but a shorter, vertical plate of bone, a metapophyses, m, of nearly the same 

 longitudinal extent as the true spine, is developed on each side of, and parallel with its 

 base ; the height of these metapophyses in the third sacral vertebra is three inches and 

 a half; they incline obhquely outwards, like the metapophyses in the dorso-lumbar 

 vertebrae of the armadillos, and evidently tend to strengthen the connection between 

 the sacral part of the trunk, and the pelvic base of articulation of the hind limbs. The 

 spinous process begins to expand longitudinally, and when nearly opposite the 

 summit of the metapophyses, is joined by vertical suture with the similarly expanded 

 neighbouring spines, so that the sacrum is crowned by a strong continuous vertical 

 longitudinal ridge of bone, at least along the first four vertebrae ; the broad spine of 

 the fifth being rounded ofi" anteriorly, and separated by a narrow interspace from that 

 of the fourth. Besides this modification of the spine, and the more normal position of 

 the neural arch, the diapophysis, d, of the fifth anchylosed vertebra, resumes its more 

 ordinary shape, and it is supported by two converging ridges of bone from the side of 

 the neural arch below. The origin of the metapophysis, ;), of the first sacral is 

 placed higher than in the three middle ones, in which the several peculiarities of 

 structure above described are most strongly marked. 



Tlie specimens of sacrum of the Megalosaurus in the British Museum, and that of 

 the Geological Society, present the same structure as that above described in the 

 original specimen at Oxford. Part of the fifth sacral vertebra is wanting in the 



