of the Tail in IchtJiyosauria. 437 



depth, so that the centrums speedily become deeper than 

 wide. This indicates lateral movement within the caudal fin 

 itself, and that inference is supported by the interarticular 

 borders of the centrum continuing rounded in examples 

 1t^ inch deep and 1 ,V inch wide. In all the earlier vertebrse 

 of this series the measurement along the neural canal is 

 rather longer than upon the ventral border. 



In an associated series of seven (F. ii. 75-81 Sedgwick 

 Museum) the antero-posterior measurement decreases from 

 little more than -^^ to -^ inch, the depth from I7V to To inch, 

 and the width from 1 inch to y^j inch. In every vertebra 

 the posterior articular surface is appreciably smaller than the 

 anterior end. The rapid decrease in size appears to indicate 

 that the caudal fin was short and supported by few vertebra?, 

 peihaps fewer than twenty. Many of the smaller vertebrae 

 have sharp margins to the centrum, as though the extremity 

 of the tin was nearly rigid. 



In these vertebras the neural canal is at first defined by an 

 elevated lateral border, but after a time this contracts from 

 front to back into a median process which is directed outward 

 and shows no indication of having supported neurapophyses. 



Subsequently, in the * Aves, Ornithosauria, and lieptilia * 

 (1869), I gave some account of an Ichthyosaurian skeleton 

 from theOxford Clay of Woodstone Lodge, near Peterborough, 

 in which seven caudal vertebras are present with convex 

 margins to the centrum (/. 13-19) rapidly diminishing in size. 

 The rib-facet is last seen on /. 15 (p. 113), so that the last 

 true caudal vertebras appear to show more than usual flexibility 

 before the caudal rib is lost, and the four flexible pivot-ver- 

 tebras occur which are anterior to the caudal fin. Only eleven 

 of the fin series are preserved, which resemble compressed 

 dorsal vertebras of a Plesiosaur. The smallest is half an inch 

 in diameter. In a well-preserved skeleton in the Sedgwick 

 Museum from the Oxford Clay of Whittlesea there are four 

 modified Centrums anterior to the caudal fin, followed by 

 fifty-one centrums with smooth rounded external surfaces 

 which have the same generalized character as other examples. 

 But the series is imperfect, and five or six or more may be 

 missing. 



In January 1889 Mr. A. N. Leeds, F.G.S., submitted to 

 nie the extremity of the tail of an Oxford Clay Ichthyo- 

 saurian which I believed to be Ophthalmosaurus. On that 

 specimen 1 made the following note, which shows the ver- 

 tebras supporting the tail to be substantially similar to other 

 specimens i'rom the Oxford Clay : — 



As the caudal vertebras ap[)roach towards the extremity of 

 An7i. cC; Ma<f. X. Hist. t>er. 8. Vol. i. 29 



