AQUATIC MAMMALS 



that locomotion was probably accomplished chiefly by the paddles. In 

 longipinnate sorts such as Ichthyosaurus, however, he claimed that the 

 upper lobe of the tail was immovable and together with the dorsal fin 

 functioned as a rudder. It is impossible that in such a highly specialized 

 form one-half of the tail could have acted efficiently as a rudder and 

 the other half as a primary propeller. Undoubtedly as the upper half 

 of the tail approached the lower half in size and form there was a 

 stiffening of the caudal tissue, just as in whales, toward the ultimate 

 goal that the caudal tendons operated from the base of the tail rather 

 than its lower tip, enabling the upper lobe to impart almost, if not 

 quite, as much propulsive force as the lower half. Any asymmetry in 

 this force could then have been equalized by tilting of the flippers. 



The second class into which I have divided the tails of aquatic mam- 

 mals is as follows : 



II. Tails flattened in the vertical and broadened in the horizontal 

 plane. In the first group below are included all those genera in which 

 the tail has been flattened vertically, and in the second, those in which 

 the tail is still terete but which may be expected eventually to develop 

 a vertically flat tail. 



Tail flattened Tail round 



Ormthorhynchus Mustek (the aquatic forms) 



Lutrinae (part) Lutrinae (part) 



Enhydrinae 

 Pinnipedia 

 Castor 

 Sirenia 

 Cetacea 



There are two members of the above groups which I am unable to 

 discuss with any great feeling of certainty. These are Ormthorhynchus 

 and Castor. It is not difficult to determine the economic value to them 

 of their caudal equipment, but it is very puzzling to envision the process 

 by which the specialization was initiated. The case of the platypus is 

 of lesser moment, perhaps, It is such an anomalous beast in so many 

 ways; we are entirely ignorant of the ancestral type, and it is the only 

 aquatic mammal with a sizable tail that swims chiefly by means of the 

 fore feet. Hence we must be satisfied with assigning the proper func- 

 tion to its tail without attempting to visualize the successive steps through 

 which it originally passed. According to all reports the tail is now 

 used as a rudder, and for the purpose of keeping the head depressed 



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