AQUATIC MAMMALS 



have gained the impression that this use is not invariably so much for 

 the purpose of speeding progression as it is for maintaining proper 

 posture in the water. Frequently when the greatest speed is attained the 

 limbs are not used at all. It is highly probable that at one time in the 

 past the tail of the otter was a less efficient swimming organ than is now 

 the case, and undoubtedly the limbs then constituted the chief propul- 

 sive apparatus, as now in the mink. But in all generalized mustelids 

 the body is long and sinuous and the legs short. Hence, body motions, 

 such as arching of the back, are important in terrestrial locomotion and 

 it is natural that a similar use was made of the body and tail when the 

 animal first took to the water. Because of this and the shortness of the 

 limbs it is not improbable that the latter did not respond so readily to 

 the stimulus for aquatic modification of the extremities, or else that 

 this stimulus was not as strong as in the case of most other mammals, 

 so that the tail more readily gained the ascendancy. 



The hind feet of otters (fig. 43) vary greatly from an unwebbed con- 

 dition to one in which the interdigital membranes are broad and full. 

 But as nothing is known about the finer points of the swimming meth- 

 ods of these less familiar sorts the subject cannot be pursued further. 



One would hardly expect much change in the otter as long as it fol- 

 lows its present mode of life. If it should increase in size and take to 

 coastal waters then its tail should broaden into a more efficient propeller 

 and its hind limbs tend to become reduced as it may become more inde- 

 pendent of the land. Eventually its pectoral limbs should assume the 

 form of equilibrating paddles and it is likely that superficially at least it 

 should finally become very much the same sort of beast as is the ceta- 

 cean. 



The subject of the sirenian pelvic element is one which I approach 

 with considerable hesitancy. Flower (1876) has said, the pelvis is ru- 

 dimentary, composed in the dugong of two slender bones on each side, 

 placed end to end, commonly ankylosed together. He considered that 

 the upper, attached to the vertebral column by a ligament, represents 

 the ilium and the lower the ischium, or ischium and pubis combined. 

 He also stated that although there was a vestigeal femur in Halitheriuni 

 this bone is not represented in any living member of the order. But 

 other observers, possibly working with different species, have reported 

 otherwise. The innominate of the dugong is usually figured as a single, 

 slender bone, made up of two terminal elements, situated with the long 

 axis vertical, connected by ligaments dorsad with the transverse process 

 of one vertebra, and ventrad to its fellow of the opposite side. Abel 



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