202 PALAEONTOLOGY 



modifications that characterize the 1 >ony support of the tail fin 

 in fishes. The numerous caudal vertebra? of the Ichthyosaurus, 

 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 skeleton of a whale 

 would yield of the powerful horizontal tail-fin characteristic of 

 the living animal, is the depressed or horizontally-flattened form 

 of the bones supporting such fin. It is inferred, therefore, from 

 the corresponding bones of the Ichthyosaurus being flattened in 

 the vertical direction, or from side to side, that it possessed 

 a tegument ary tail fin expanded in the vertical direction. The 

 shape of a fin composed of such perishable material is of 

 course conjectural, as is the outline in fig. 68. Thus, in 

 the construction of the principal swimming organ of the 

 Ichthyosaurus we may trace, as in other parts of its structure, 

 a combination of mammalian (beast-like), saurian (lizard- 

 like), and piscine (fish-like) peculiarities. In the great 

 length and gradual diminution of the tail we perceive its 

 saurian character ; in the tegumentary nature of the fin, 

 unsustained by bony fin-rays, its affinity to the same part 

 in the mammalian whales and porpoises is shewn ; whilst 

 its vertical position makes it closely resemble the tail fin of 

 the fish. 



The horizontal ity of the tail fin of the whale tribe is essen- 

 tially connected with their necessities as warm-blooded animals 

 breathing atmospheric air : without this means of displacing 

 a mass of water in the vertical direction, the head of the whale 



Prans. Geol. s '"' 2d Bevies, rol. v., p. 511. 



