AQUATIC MAMMALS 



significance of the word "correspondingly" in the last sentence. The 

 principle is clear but the formula cannot be worked out. 



The above mode of swimming, termed the fusiform type because 

 a spindle-shaped body is the best fitted to employ it, is followed by 

 most fish, Potomogale, Sirenia, Phocidae (in seals the hind feet are 

 employed the same as a tail), and Cetacea. Almost always there are, 

 or finally will be, at least one rudder or pair of rudders or stabilators 

 disassociated from the primary organ of propulsion, and not infrequently 

 there is one or more accessory equilibrate rs (as a dorsal fin) . 



If the body and tail become disproportionately long for their diameter 

 this mode of swimming will change to the anguilliform or eel-like 

 type. The length, relative to diameter, must be sufficiently great for 

 the animal to assume an S-shaped posture, when it will then swim by 

 what constitutes, in reality, a duplication of the fusiform method of 

 swimming, slipping through the water as a snake slips through the grass. 

 This is of interest in the present connection chiefly for the reason that 

 this was undoubtedly the method of swimming employed by long- 

 tailed zeuglodonts of the Basilosaurus type. But there may be body 

 shapes and swimming methods intermediate between the anguilliform 

 and fusiform types. Some of the longer bodied, modern whales may 

 be partially anguilliform in their motions, and the Weddell seal (Lep- 

 tonychotes) certainly is remarkably serpentine in its terrestrial move- 

 ments. Then too there were short-bodied and long, anguilliform-bodied 

 zeuglodonts, and ichthyosaurs of similarly different conformation. Pre- 

 sumably the tendency should be for a shortening of an anguilliform ver- 

 tebrate because the fusiform method of swimming is capable of higher 

 speed, and the shorter body encounters less water friction. 



Propulsion by the fore limbs, and by the hind limbs in other manner 

 from that in which the hair seals (Phocidae) use these members, in- 

 volves different principles. The most specialized instance of propulsion 

 by the fore limbs among Mammalia is encountered in the sea-lions and 

 fur seals (Otariidae). The principle is much the same as that of a 

 row-boat. The latter should be fairly long for its width, for if it be 

 short and tubby it will tend to wabble rather than progress in a straight 

 line. It is evident also that the oars should operate from near the center 

 of the craft, and that the latter should taper from the middle toward 

 either end. This description fits the sea-lion, which apparently can 

 steer with ease by means of either the head and neck or the hinder end. 

 The description does not fit the platypus, however, which also swims 

 chiefly by means of the pectoral limbs. In the latter the mass anterior 



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