EXTERNAL FEATURES 



a fin, almost certainly as an aid to equilibration. It should be men- 

 tioned, however, that a dorsal fin can hardly act very efficiently as an 

 equilibrator in an animal swimming by a vertical motion of the tail (as 

 whales) as it can in one employing a horizontal motion (as most fish), 

 for equilibrators should be situated at a right angle to the direction of the 

 swimming force. In the Cetacea, therefore, the situation of the flippers 



Figure 11. Some of the extinct aquatic reptiles: (a) Mesosaurus ; (b) Elasmo- 

 saurus (plesiosaur) ; (r) Clidastes (mosasaur) ; {d) Ichthyosaurus; 

 (e) Trimacromerum (plesiosaur): (redrawn from Williston). 



renders these appendages by all odds the most efficient organs of equili- 

 bration and it seems that a dorsal fin would not be of sufficient practical 

 importance for all sorts of whales invariably to have developed it. It 

 may be mentioned that theoretically a dorsal fin would be of greater use 

 to the Phocidae in pure equilibration than the fore limbs, but these mam- 

 mals are not yet sufficiently specialized to have developed the former. 



[59] 



