662 



caudal fin seems especially required by the short-necked and stiff- 

 necked Ichthyosauri, in order to produce, with sufficient rapidity, 

 the lateral movements of the head, which were needed by those pre- 

 datory inhabitants of the ancient deep; while, in the Plesiosaurus , 

 such a fin would be unnecessary, in consequence of the length and 

 mobility of the neck ; and Mr. Owen concludes, by stating, that in 

 those skeletons of Plesiosauri in which the tail is perfect, it is 

 straight, and presents no indication of the partial fracture or bend, 

 which is so common in the tails of Ichthyosauri. 



Figures of the tails of five specimens of Ichthyosauri, now in Lon- 

 don, accompanied the Note ; the subject of which was also iUus- 

 tvated by a sixth skeleton of an Ichthyosaurus on the Table, the 

 property of Sir John Mordaunt, Bart. 



A paper was commenced, on the Primary Formations of England, 

 by the Rev. Adam Sedgwick, V.P.G.S.; Woodwardian Professor in 

 the University of Cambridge, &c. 



