43G Prof. H. G. Sceley on the Extremity 



LXYIII. — On the Extremitif of the Tail in Ichthyosauria. 

 By H. G. Seeley, F.It.8., F.G.S., King's College, Londou. 



As early as 1839 Sir Richard Owen described the small 

 laterally compressed ribless terminal caudal vertebras which 

 supported the caudal fin in the Ichthyosaurus; but the 

 caudal fin itself remained unknown till figured by Dr. Eber- 

 hard Fraa?. Owen also mentions^ in his general account of 

 the osteology of the Ichthyosaurs from the Lias, that these 

 vertebrae are preceded by three or four with the centrums 

 more compressed, and tlieir margins raised, in the region where 

 the abrupt bend or distortion of the tail usually takes place. 

 That abrupt bend was formerly regarded as a postmortem 

 condition produced by the weight of the tail-fin. The 

 specimens available in 1839 were all more or less imbedded 

 in slabs of Lias. 



In 1869, in a short account of the Ichthyosauria of the 

 Cambridge Greensand, I gave some ])articulars of vertebrae 

 from the terminal part of the tail which are free from matrix. 

 The specimens suggest that there were probably three 

 vertebrge between the caudal series supporting short caudal 

 ribs and the caudal fin-series from which ribs are absent. 

 As in the Lias specimens, these pivot-vertebrje are distin- 

 guished by their antero-posterior measurement being slightly 

 diminished ; it is less on the ventral than on the neural 

 border; the central pit is much less deeply impressed in the 

 centrum than in the earlier caudals ; the lateral margins of 

 the articular faces of the centrums are convex, and so rounded 

 as almost to meet and nearly obliterate the lateral surfaces 

 of the centrum ; but this condition is not seen on the ventral 

 margin. The largest of these pivot-vertebrse are 2^- to 2| 

 inches wide, "I-yjj to 2^ inches deep, and yV itich from front to 

 back. The measurements and conditions show that the 

 vertebrae from this point are directed downward, and that the 

 angular bend in the Ichthyosaurian tail is a natural condition 

 of downward angular flexure due to wedge-shaped form of 

 these centrums. The rounded lateral tumid borders of these 

 verteiirte show that the movement was from side to side, and 

 that the caudal fin could only be used as a rudder or steering- 

 organ when moved upon these pivot-vertebise. 



In the Cambridge Greensand species the caudal fin appears 

 to have been very short, for the centrums whicii support it 

 decrease in size very rapidly in vertical and transverse 

 measurements, though there is a slight increase in length. 

 'J'he contraction in width is more rapid than the decrease in 



