ICHTHYOPTERYGIA. 



221 



of fishes, in the Labyrinthodonts, and in the existing perenni- 

 branchiate Batrachia, but not in 

 any of the whale or porpoise 

 tribe. 



With the abov T e modifications 

 of the head, trunk, and limbs in 

 relation to swimming, the struc- 

 ture of the tail corresponds. The 

 bones of this part are more nu- 

 merous than in the Plesiosaurs, 

 and the entire tail is conse- 

 quently longer ; but it does not 

 shew any of those modifications 

 that characterize the bony sup- 

 port of the tail-fin in fishes. The 

 caudal vertebrae of the Ichthyo- 

 saurus gradually decrease in size 

 to the end of the tail, where they 

 assume a compressed form, or are 

 flattened from side to side, and 

 thus the tail, instead of being 

 short and broad as in fishes, is 

 lengthened out as in crocodiles. 



The very frequent occurrence 

 of a fracture of the tail, about 

 one-fourth of the way from its 

 extremity, in well preserved and 

 entire fossil skeletons, is owing to 

 that proportion of the end of the 

 tail having supported a cutaneous 

 and perishable caudal fin * The 

 only evidence which the fossil 



* Trans. Geol. Soc, 2d series, vol. v., 

 p. 511. 



