222 PALAEONTOLOGY. 



skeleton of a whale would yield of the powerful hori- 

 zontal tail-fin characteristic of the living animal, is the de- 

 pressed 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 tegu- 

 mentary 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. 89. Thus, in the 

 construction of the principal swimming organ of the Ichthyo*- 

 saurus we may trace, as in other parts of its structure, a combi- 

 nation 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 horizontality of the tail-fin of the whale tribe is 

 essentially connected with their necessities as warm-blooded 

 animals, air-breathers needing ready access to atmospheric 

 air ; without the means of displacing a mass of water in 

 the vertical direction by the tail, the head of the whale 

 could not be brought with the required rapidity to the sur- 

 face to respire : but the Ichthyosaurs, not being warm- 

 blooded or quick breathers, would not need to bring their 

 head to the surface so frequently or so rapidly as the 

 whale ; and moreover, a compensation for the want of hori- 

 zontality of their tail-fin was provided by the addition of a 

 pair of hind paddles, which are not present in the. whale tribe. 

 The vertical fin was a more efficient organ in the rapid cleav- 

 ing of the liquid element, when the Ichthyosaurs were in pur- 

 suit of their prey, or escaping from an enemy. 



The general form of the cranium of the Ichthyosaurus 



