WEALDEN FORMATIONS. 31 



The anterior articular extremity in one of these vertebrae makes an approach to a 

 plane surface, being slightly concave transversely below, and very slightly convex 

 above ; vertically it is very slightly convex ; the depth of the posterior concave surface 

 at the centre is 9 lines. The general contour of the centrum has begun to change 

 from the circular to the subquadrate, which latter figure is more decidedly expressed 

 in the anterior caudal vertebrae of Cetiosauriis hrevis (Tab. X). 



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 degree of smoothness, 

 and is pierced here and there by moderate-sized vascular foramina. The neural canal 

 makes a slight depression in the upper part of the centrum ; in the Iguanodon it is 

 encompassed by the bases of the neurapophyses. The transverse diameter of the 

 neural canal is 1 inch, which small dimension satisfactorily distinguishes the present 

 enormous vertebra from those of the mammiferous class, viz., the Cetacea, to which 

 in other respects it has the greatest similitude. The antero-posterior diameter of the 

 base of the neurapophysis is 2 inches. 



The four anterior caudal vertebrse in the Mantellian Collection, which are here 

 assigned to Cetiosaurus hrevis, slightly increase in antero-posterior diameter, as is 

 the case with 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, however, that, even in the present short segment 

 of the tail, with the slight increase 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 vertebrse recede from the trunk. The third of these vertebrae is figured of the 

 natural size in Tab. X. 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 (Tab. X, h, h) of this inferior surface. The articular surfaces at both ends 

 of the centrum are now slightly 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 the 

 surface.* The sides of the centrum at the upper half are concave both lengthwise 

 and vertically, forming a wide depression below the transverse process ; the middle 

 part of the side begins to stand out and divide the upper from the lower lateral 

 concavity, which character, being more strongly developed in the hinder caudal 

 vertebrae, gives the rhomboidal or hexagonal form.f The lower half of the side of 



* The same modification of tlie articular extremities occurs in tlie caudal region of the vertel)ral 

 column of the Plesiosaurus. See ' Report,' part i, 'Trans. Brit. Assoc' 1839, p. 58. 



•f It is one of these posterior caudals of the Cetiosaurus which is figured as the type of the " second 

 vertebral system" in the ' Geology of the South-east of England,' p. 296, fig. 2. 



