SWIMMING 



(b) Propulsion by means of pectoral appendages 



Pisces (many fish use the pectoral fins for slow propul- 

 sion; the skates and rays (Batoidea) exclusively) 



Reptilia 



Chelonia (the marine turtles) 



Aves (penguins and such birds as "fly" under water) 



Mammalia 

 Monotremata 

 Pinnipedia (Otariidae) 



(c) Propulsion by pelvic appendages 

 Reptilia (Salientia frogs and toads) 

 Aves (all birds that swim with the feet) 

 Mammalia 



Marsupialia (Chironectes) 



Insectivora (practically all aquatic forms save Potomo- 

 gale and Limnogale) 



Carnivora (Enhydrinae) 



Pinnipedia (Phocidae) 



Rodentia (all aquatic forms chiefly, most of them ex- 

 clusively) 



(d) Propulsion by undulations of a longitudinal fin 



Pisces (an important, although possibly not the most 

 effective, means of locomotion of such forms as Amia- 

 tus and Gymnotus, and exclusively of the Hippocampi- 

 dae or sea horses) 



(e) Propulsion by vibration or undulation of the tail tip 

 Pisces (used by many fish at slow speeds, and by such 



as are incased in an unyielding body covering, as the 

 Ostraciidae) 



In some cases a certain vertebrate may swim by a combination of the 

 above methods, and at times, especially in the case of certain extinct rep- 

 tiles, it is extremely difficult to determine the precise method of swim- 

 ming. 



Although beset with several difficulties which are at present insur- 

 mountable, some of the principles underlying the aquatic progression 

 of mammals may be discussed with confidence. 



[13] 



