ADAPTATIONS TO SPECIAL ENDS 133 



A more advanced and specialised type of skeleton is pre- 

 sented by the paddles of the plesiosaurs. Here the fore and 

 hind paddles are nearly of a size and the humerus (or upper 

 bone) in the one and the femur in the other, although much 

 expanded inferiorly, retain the characters of " long bones ". 

 The radius and ulna in the fore-paddle and the tibia and fibula 

 in the hind pair have, however, altogether lost this character, 

 being much shortened and laterally expanded, so that they are 

 practically similar to the bones of the carpus and tarsus which 

 respectively come below them. Furthermore, although the 

 bones of the metacarpus (in the fore-limb) and metatarsus (in 

 the hind-paddles) and of the digits (in both) retain much the 

 normal contour and proportions, yet the number of phalangeal 

 bones in each digit is increased beyond the ordinary. If, 

 however, we trace the plesiosaurs down into strata of Triassic 

 age we find all the bones of a more normal type, this being 

 especially noticeable in the case of the radius and ulna and the 

 corresponding elements in the hind limb. These early sauro- 

 pterygians were, indeed, at most only partially aquatic, and 

 certainly not marine. 



Of a totally different and more specialised type is the limb- 

 skeleton of the ichthyosaurs (Ichthyopterygia), in which the 

 front-paddles were approximately double the size of the hind 

 pair. In the Liassic members of this group with the most 

 complicated type of paddle the humerus, although retaining in 

 a modified degree the character of a " long-bone " has become 

 greatly shortened and thickened ; but the whole of the bones 

 below this have come to resemble those of the carpus in general 

 character, so that they form a continuous mosaic-like pavement, 

 with no separation of the digits. Not only has the number of 

 individual bones in each digit been largely augmented, but the 

 number of digits has been increased above the normal five, 

 apparently by the splitting, or forking, of some of them. In 

 none of the bones of the digits is there any trace of a division 

 into a "shaft," and two terminal "heads" or expansions; that 

 is to say, they have entirely lost all traces of evidence that they 

 correspond to " long bones," like those in the paddles of the 

 turtles or in the feet of terrestrial reptiles. 



If, however, we trace the ichthyosaurs down into the Trias, 

 we find that their paddles show distinct evidence of derivation 



