ON BRITISH FOSSIL REPTILES. 107 



upward, forward and backward, is joined by vertical suture to similar ex- 

 pansions of the contiguous neurapophysis, an-d terminates above in a ridge 

 of bone, at right angles to the suture ; this ridge, with those of the other 

 neurapophyses, extends longitudinally above the transverse processes the 

 whole length of the sacrum, and forms the margin of the platform from which 

 the spinous and accessory processes are developed. The platform is further 

 supported by a compressed ridge of bone extended from the upper part of 

 the transverse process, 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, 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 accessory spines in the third sacral vertebra 

 is three and a half inches. The true spinous process begins to expand longi- 

 tudinally, and when near tne summit of the accessory spines, is joined by ver- 

 tical siiture with the similarly expanded neighbouring spines, so that the sa- 

 crum is crowned by a strong continuous vertical longitudinal ridge of bone, at 

 least along the four first vertebrae ; the broad spine of the fifth being rounded 

 off 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 of the fifth anchylosed vertebra, the origin of the transverse pro- 

 cess has been suddenly raised to the level of the spinal platform, and it is sup- 

 ported by two converging ridges of bone from the side of the neural arch be- 

 low. The origin of the transverse processes of the first sacral is also placed 

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

 ture above described are most strongly marked. 



The specimens of sacrum of the Megalosmirus 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 ver- 

 tebra is wanting in the specimen from Dry Sandford. The rest are cha- 

 racterised by the same smooth and polished surface, rich brown colour, con- 

 traction of the middle of the body, its cylindrical form transversely, and the 

 longitudinal fossa below the annular part. The length of this series is one 

 foot six and a half inches ; the second and third sacral vertebrae are rather 

 shorter than the rest. The first sacral vertebra, which was not anchylosed 

 to the last lumbar, gives the following dimensions : — 



In. Lin. 



Antero-posterior diameter of centrum 5 



Vertical diameter of anterior articular end .... 4 

 Transverse diameter of anterior articular end ... 4 6 

 The neural arch seems not to have been co-extensive in length with the cen- 

 trum, but rests on its anterior three-fourths. A strong and short transverse 

 process extends obliquely upwards and backwards from each side of the arch ; 

 the antero-posterior diameter of the base of this process is two inches, its ver- 

 tical diameter one and a half incii. In the second sacral vertebra the neural 

 arch ha.s moved forward upon the interspace between the first and second 

 sacral bodies, and developes from a lower part of its base a stronger, thicker 

 and longer transverse process, directed outwards and forwards. The third 

 neural arch has its base transferred directly over the interspace of the second 

 and third centrums ; the diameters of the base of its transverse processes are 

 three inches and two and a half inches : they incline slightly backwards. The 

 fourth neural arch descends lower down upon the interspace between the third 

 and fourth centrums. The fifth neural arch, as in the Oxford specimen, ex- 

 tends a little way across the interspace between the fourth and fifth centrums, 



