ON BRITISH FOSSIL REPTILES. 131 



interspace of these anterior and posterior neurapophysial surfaces is formed 

 by a smooth oblique groove, connecting the smooth surface of the spinal canal 

 with that of the free lateral surface of the vertebra, and indicating the place 

 of exit of the sacral nerves, which is necessarily in this unusual situation, be- 

 cause the ordinary holes of conjugation must have been obliterated by the 

 impaction of the bases of the neurapophyses between the contiguous extre- 

 mities of the bodies of the sacral vertebrae. 



The anterior and posterior articular extremities of the present interesting 

 fossil equally bespeak the peculiar character of the sacral vertebrae of the 

 Dinosauria. They are impressed by coarse straight ridges and grooves ra- 

 diating from near the upper part of the surface, like those on the correspond- 

 ing part of a cetaceous vertebra when the epiphysial articular extremity is 

 removed. These inequalities are here, doubtless, preparatory to that anchy- 

 losis by which the sacral vertebrae are compacted together in the mature Di- 

 nosaurs. 



In. Lines. 



The length of this vertebra 2 10 



The height 2 6 



The breadth of anterior articular end , 3 



The breadth of middle 2 2 



Antero-posterior diameter of anterior costal surface . 1 7 

 Antero-posterior diameter of posterior one .... 1 



Breadth of spinal canal 1 5 



Breadth of canal of sacral nerve 4 



From its separated condition, the body of the sacral vertebra here described 

 must have belonged to a young Dinosaur of a size far exceeding that of the 

 HylcBOsaurus. It is obviously very distinct in form from the sacral vertebrae 

 of the Megalosaurus. No other reptile than one belonging to the order cha- 

 racterized by the peculiar structure of the sacrum already described, could 

 have yielded a detached vertebral centrum with the remarkable modifications 

 of the one under consideration. The modifications detected in the entire 

 sacrum of the Iguanodon in Mr. SauH's collection, justify the reference of the 

 vertebra above described to the sacrum of a young Iguanodon. 



Caudal Vertebrce. — These are distinguished by the single haemapophysial 

 surface at each end of the narrow inferior surface of the centrum. The sides 

 of the centrum are flat, or even slightly concave in the vertical direction, 

 though less so than in the antero-posterior direction. In a caudal cen- 

 trum, for example, in the Mantellian Collection, measuring ^ inches in 

 length, and 5 inches 4 lines in depth at the middle of the side, if a pencil 

 be laid vertically along that part, an interval of between 1 and 2 lines sepa- 

 rates its middle part from the bone. Those great Wealden vertebrae which, 

 on the contrary, have the middle of the side of the body prominent, and the 

 lower half only converging towards the under surface, are from the middle 

 and posterior part of the tail of the Cetiosaurus. The posterior terminal ar- 

 ticular surface is rather more concave than in the dorsal vertebrge ; but the 

 difference is by no means so marked as in the plano-concave vertebrae of the 

 Cetiosaurus. The transverse processes of the anterior caudal vertebrse are 

 comparatively short, but strong, and are continued from the base of the neur- 

 apophysis. 



The haemapophyses, or chevron bones, are not anchylosed to the centrum, 

 but articulate with the interspaces of the vertebrae ; in a few of the anterior 

 ones to two distinct but closely approximated surfaces on each contiguous 

 vertebra, but in the rest of the caudal vertebrae to a single oblique triangular 

 surface on each of the contiguous extremities of the centrum; the haemapo- 



k2 



