42 



that the skull when attached to the vertebral cokimn is 

 directed markedly downwards (see fig. 17). 



The anterior notochordal space in most of Ihe trunk 

 vertebrae (except the atlas) is perceptibly deeper than the 

 posterior, so that the notochordal canal is situated nearer 

 the posterior than the anterior face of the centrum. This 

 is more marked in some vertebrae than in others. 



Atlas (figs. 10 and IT). — Body compressed from before 

 backwards. Xotochordal canal (N.C.) much nearer dorsal 

 than ventral surface. Bears two large cartilage capped 

 facets {C.F.) for articulating with the paroccipital con- 

 dyles, of which the left is perceptibly larger than the 

 rie-ht. The anterior face of the centrum also articulates 

 with the single occipital condyle on the basi-occipital, the 

 connection of the skull with the vertebral column by 

 means of 3 condyles being therefore very strong. Unlike 

 all the other vertebrae, except about the last 5, the neural 

 arch of the centrum is only perforated by one foramen on 

 each side for the second spinal nerve. There is no trans- 

 verse process, and only one rib, which belongs to the series 

 of accessory ribs or intermuscular bones [A.R.^), and is 

 attached to its vertebra higher up than any of the others, 

 articulating at the junction of the neural arch with the 

 centrum (figs. 10, 17). As in all the other vertebrie 

 (although the tendency is faint in the posterior caudals), 

 and as first described by Traquair, the atlas is markedly 

 asymmetrical, the neural spine being directed towards the 

 eyeless side. The asymmetry here is obviously an adapta- 

 tion to the habit of the animal in lying on its eyeless side. 

 Superficially this side is practically flat, whilst the ocular 

 side is convex. The asymmetry of the vertebrae, there- 

 fore, tends to a flattening of the eyeless side and an arch- 

 ing of the ocular side (cp. figs. 11 and 18). The neural 

 spine itself is in the form of a rolled plate forming a hollow 



