1G0 MAEIXE EEPTILES OE THE OXFORD CLAY. 



On the inner side the articular is supported by the angular and by the prearticular, 

 Avhich covers its inner face ; externally it unites with the surangular. 



Vertebral Column (text-figs. 61-66). — In general structure the atlas and axis 

 of Metriorliynclms (text-fig. 61) are very similar to those of Steneosaurus, but differ in 

 certain points to be noted below. These vertebrae have been described in detail by 

 several writers (Hulke*, Jaekelf, ArthaberJ, and others). 



The first subvertebral wedge-bone (a.iv.b.) (hypocentrum of atlas) forms the lower 

 portion of the cup for the occipital condyle, extending up to about the middle of the 

 sides, where its upper angles articulate with the lower ends of the bases of the neural 

 arch. Posteriorly it unites with the obliquely truncated lower border of the odontoid 

 (od., centrum of atlas). The lateral borders are truncated by a pair of elongated 

 facets, looking outwards, downwards, and backwards, for union with the head of the 

 first rib. The two halves of the neural arch (at. a.) do not seem to have united in the 

 middle line, and in no specimen is a proatlas preserved, although that structure was no 

 doubt present. Each half of the arch is produced backwards into a prominent posterior 

 zygapophysis for union with the arch of the axis. Ventrally the enlarged bases of 

 the arch unite with a broad surface of the odontoid, extending backwards to the anterior 

 edge of the axis. Anteriorly they form the superior lateral portions of the atlantal 

 cup. On the sides of these basal portions of the neural arch of the atlas there are, in 

 a few cases, well-marked diapophysial prominences. This is especially well shown in 

 an atlas and axis (R. 2054) figured by Hulke (Proc. Zool. Soc. 1888, pi. xviii. fig. 1). 

 At the same time the proximal end of the first rib shows no trace of division into head 

 and tubercle. Perhaps, as in the case of the rib of the axis in Steneosaurus, the 

 tubercle was represented by a ligament joining the diapophysis. The partial 

 persistence of the diapophysis in the atlas probably indicates that the rib was 

 originally double-headed. 



The odontoid (od.), which in adults is fused with the axis, presents some peculiar 

 features. On its ventral side it is divided by a median groove into two lateral 

 prominences ; the groove (f.) is continued up the anterior face of the bone to about its 

 middle point. Probably this groove represents the suture described by Jaekel (loc. cit.) 

 as occurring in the odontoid of M. jaekeli (1 M. super ciliosum), and may indicate that the 

 odontoid was originally a paired structure — that is to say, according to the view here 

 adopted, is formed by the union of the pleurocentra of the first vertebra. Anteriorly 

 the odontoid forms the middle and upper part of the atlantal cup ; above it is gently 

 concave from side to side, forming the floor of the neural canal. Laterally it is largely 



* Proc. Zool. Soc. 1S8S, p. 418. 



t Zeitschr. deutscli. geol. Gesellsch. vol. hi. (1904) p. 109. 



1 Beitr. Palaont. Oesterr.-Ung. vol. six. (1906) p. 295 ; in this paper a summary of ths various 

 views as to the nature of these vertebra is given. 



