ON BRITISH FOSSIL REPTILES. 97 



were nipped in between two shallow depressions ; its base ascends obliquely, 

 and grows thicker to the posterior part of tlie neural arch. The summit is 

 not entire. 



The height of this dorsal vertebra to the posterior origin of the spinous 

 process is 9^ inches : from the base of the neurapophysis to the upper part 

 of the transverse process, measures 3 inches. 



No. " 1^ " in the Mantellian Collection, (" Vertebra of Iguanodon, 8 inches 

 in diameter," MS. Catalogue), may have actually presented that dimension 

 when entire, for even now, not allowing for the margin of the posterior arti- 

 cular surface which has been broken away, it measures 7 inches across that 

 surface. This remarkable specimen, which may probably have afforded the 

 type of the 'third or plano-concave' vertebral system, in the summary of the 

 vertebral characters of the Wealden reptiles given by Dr. Mantell in his 

 ' Geology of the South-east of England*,' and which accords best with the 

 characters assigned by M. H. von Meyer to the vertebrae of the Iguanodon\, 

 presents, in fact, in a striking degree, those of the vertebrae of the Cetiosaurus, 

 and belongs to a more posterior part of the dorsal region, perhaps to the loins, 

 of the same individual, certainly to one of the same species, as the vertebra 

 (No. 2133) last described. 



The anterior articular extremity makes the same approach to a plane sur- 

 face, being slightlj^ concave below and very slightly convex above : the depth 

 of the posterior concave surface at the centre is 9 lines. The general con- 

 tour of the centrum has begun to change from the circular to the subqua- 

 drate, which latter figure is more decidedly expressed in the anterior caudal 

 vertebrae of the Cetiosaurus brevis. 



The upper half of the sides of the centrum are more concave in the axis 

 of the vertebra than in No. 2133. The free surface presents the same de- 

 gree of smoothness, and is pierced here and there by moderate-sized vascu- 

 lar foramina. The spinal canal makes a slight depression in the upper part 

 of the centrum : in the Iguanodon it is encompassed by the bases of the neu- 

 rapophyses. The transverse diameter of the spinal canal is 1 inch, Avhich 

 small dimension satisfactorily distinguishes the present enormous vertebra 

 from those of the raammiferous class, viz. the Cetacea, to which in other re- 

 spects it has the greatest similitude. The antero-posterior diameter of the 

 base of the neurapophysis is 2 inches. 



The four anterior caudal vertebras in the Mantellian Collection, which are 

 here assigned to the Cetiosaurus brevis, slightly increase in antero-posterior 

 diameter, as is the case in the Cetiosaurus medius, as they recede from the 

 trunk, which seems to indicate that the present gigantic marine Saurian must 

 have had a capacious and bulky trunk, but propelled by a longer and more 

 Crocodilian tail than in the modern whales. It is sufficiently evident, how- 

 ever, that, even in the present short segment of the tail, with the slight in- 

 crease of length, there is a diminution of height and breadth of the centrum, 

 and a still more obvious subsidence of the neural arch, as the vertebrae recede 

 from the trunk. As compared with the dorsal vertebrae, the chief change of 

 form is the subquadrate contour produced by a lateral extension and flattening 

 of the lower surface of the centrum, which is more essentially distinguished 

 by four haemapophysial articular surfaces, two at the anterior and two at the 

 posterior margins of this inferior surface. The articular surfaces at both ends 

 of the centrum are now concave ; and the anterior one, which was nearly flat 

 in the dorsals, is here the deepest ; it is one inch deep at the upper third of 



* 8vo, 1833, p. 296, fig. 3. t Palscologica, p. 212. 



184.1. H 



