u 



MAEINE EEPTILES OF THE OXFOED CLAT. 



anterior border of the spine runs down as a strong crest on the inner side of the zyga- 

 pophysis, which in this region was thus completely separated from its fellow of the 

 opposite side. The posterior zygapophyses also seem to have been separated from one 

 another, though not so clearly. 



In the posterior cervical and dorsal regions the neural arches show no signs of 

 division into two halves, and at the same time the anterior and posterior zygapophyses 

 become confluent in the middle line, so that there is only a single anterior, and a 

 single posterior median articular surface (text-fig. 30), as in the later species of 

 Ichthyosaurus. The neural spine is a high plate of bone, sloping a little backwards 

 and abruptly truncated almost at right angles at its upper end, which bears a shallow 



Text-%. 30, 



Neural arch of a dorsal vertebra of OpMialmosaurus : A, from front ; B, froui side ; 



C, from back. (E. 2137, | nat. size.) 



a.z., anterior zygapophysis ; n.c, neural canal ; «./., facets for union with centrum ; 



n,sp., neural spine ; p.z., posterior zygapophysis. 



groove, apparently indicating that in life it was tipped with a crest of cartilage. The 

 neural arch is high, but not very wide ; the ends of the pedicles are deeply grooved, 

 showing that they were capped with cartilage, the persistence of which, as above 

 mentioned, fully accounts for the almost invariable separation of the arch from the 

 centrum in the fossils. 



In the middle caudal region the neural canal becomes very small and the arch very 

 small, the spine being scarcely at all developed (text-fig. 27, A, B) and the zyga- 

 pophyses entirely wanting. At the region where the downward bend takes place 

 (text-fig. 27, C, D) the arches are stout fl-shaped bones, the upper part of which 

 forms a thick blunt spine projecting a little backwards. A little behind the bend 



