﻿WEALDEN FORMATIONS. 



3 



the non-articular surface is so coucave lengthwise as to give the appearance of the 

 centrum being constricted between the terminal articular surfaces. These are almost 

 flat. 



In one trunk-vertebra, the sides of the centrum converge to a carinate inferior surface. 

 In another (Plate I, figs. 1 — 3) that surface is less narrow (ib., fig. 2). In both 

 the suture of the neural arch is traceable, but the arch has remained attached : it shows 

 a small facet (fig. 1, p) for the head of the rib at the fore part of the base of the neur- 

 apophysis. A horizontal (diapophysial) ridge (ib. d) extends from the prezygapophysis to 

 the upper surface of the postzygapophysis, broadening as it recedes. The neural spine is 

 compressed, but rises from nearly the entire length of the neural arch. The outer 

 surface of the centrum is compact, smooth, and glistening; and on making a vertical 

 longitudinal section the more definite generic character of the large chondrosal vacuity 

 was exposed, as in fig. 3, ch, 3. 



In the series of five vertebras, including the three hinder lumbars and the sacrum (ib., 

 fig. 4), the costal surface has been transferred to the diapophysial ridge, d, which now extends 

 outward from a contracted base midway between the zygapophyses, the terminal articular 

 surface being 'supported by a lower buttress-like ridge, /. The under surface of the 

 centrum is here broader than in the preceding vertebra, and is transversely rounded : 

 the carinate character in the dorsal vertebras, giving space to the abdominal cavity, has 

 here disappeared. In some of the present series the deeply concave side of the centrum has 

 yielded to pressure, and the compact outer wall has been fractured and pressed in upon 

 the chondrosal or quasi medullary cavity. In the last lumbar vertebra the diapophysis, 

 depressed and subelongate, shows a narrow costal surface, d' } for a small or short ' false 

 rib.' 



The two hindmost vertebras in this series of five are sacral (« i, « 2). They have the 

 crocodilian character of limited number, and the non-dinosaurian character of retaining 

 their neural arch in normal junction with the centrum. The doubt expressed as to the 

 ordinal affinities of Poihilopleuron, 1 in my ' Report,' is here dispelled. The diapophysis, 

 short, but broad and deep ( s 1, d), terminates in a large flattened semi-oval surface for the 

 sacral rib. The corresponding surface upon an equally large diapophysis in the second 

 sacral has rather less vertical extent^ 2, d). The centrums appear to have coalesced, but 

 the primitive line of separation of the terminal expanded surfaces is traceable. 



The neural spines are broken away in all this series of vertebrae, but their narrow 

 elongate bases indicate the same character as in the detached more anterior vertebra 

 from a smaller individual (figs. 1 and 3, ns). 



The two caudal vertebras (figs. 5 — 8) are from the terminal part of the tail where both 

 transverse and spinous processes have disappeared. The low neural arch has coalesced 



1 " Subsequent discoveries may prove it to belong, like the Megalosaurus, to the Dinosaurian order ; 

 but, as the Poikilopleuron is, at present, known, it seems to have most claim to be received into the ccelo- 

 spondylian family of the Crocodilian order," ' Rep. Brit. Assoc.,' 1841, p. 85. 



