438 Prof. H. G. Seeley on the Extremity 



the region which carries caudal ribs the centrum rapidly 

 becomes smaller, and the outline of its articular face is 

 hexagonal. In the last three or four centrums the single rib- 

 facet rises from the inferior position it has previously occupied 

 towards the middle of the side of the centrum. It is a trans- 

 verse tubercle extending between the anterior and posterior 

 faces, with its articular portion towards the anterior margin. 

 The neural canal remains large and wide, and, as in all 

 other vertebra, wider behind than in front. The margin of 

 the intercentral articular surface is sharp anteriorly and 

 convex posteriorly, and in harmony with this character the 

 posterior surface becomes flattened, with a marked central 

 concavity, veiy unlike the typical intervertebral cupped 

 condition of an Ichthyosaur. Indeed, the facies of the 

 centrum in the latest of these rib-bearing caudal vertebrae is 

 essentially cetacean both in external form and in the manner 

 in which the external lateral surfaces are perforated by 

 innumerable close-set vascular foramina. In the last pivot- 

 vertebra but one the base of the centrum develops on the 

 anterior border, in the usual position of chevron bones in 

 other animals, two tubercles which do not appear to be 

 separate granules. In the centrum, which measures ante- 

 riorly 4*5 cm. deep, 5'2 cm. wide, and is 2"1 cm. from front 

 to back, the transverse measurement over these granules is 

 2"5 cm. The last centrum of this series might almost be 

 described as procoelous, the posterior surface being a well- 

 rounded hemisphere with a small central pit. This ball is 

 somewhat wider than deep, and occupies half the length of 

 the centrum, which is 2"6 cm. long. The anterior surface is 

 concave from above downward and less concave transversely, 

 with a conspicuous central concavity which preserves the 

 remnant of the Ichthyosaurian intervertebral type. This 

 surface measures 4*7 cm. vertically, and 3*4 cm. transversely, 

 so that the proportions of the centrum have now changed. 

 This is chiefly owing to the development of the chevron 

 granules into an inferior process. In the middle height of 

 the sides are small facets adjacent to the posterior articular 

 border, which, I suppose, supported the last pair of caudal 

 ribs ; they were probably very short. 



The succeeding vertebra is of very irregular form, higher 

 than wide, with the anterior surface flattened, with an elevated 

 articular border and a central conical impression and a slight 

 ridge midway between the central cup and the external 

 border. The posterior surface is convex, but irregular and 

 rugose. This is the pivot-vertebra of the caudal fin. 



