THE TAIL 



what similar to those that the hippopotamus has experienced. One would 

 naturally envision this ancestral type as swimming by alternate strokes 

 of at least the posterior, if not all four, limbs, while an inadequate tail 

 trailed behind. If this had been the case the stimulus would have been 

 chiefly for a transversely flattened tail. In order for the tail to have at- 

 tained the shape that we now find it, it is probable that either one of two 

 things was an original requisite: (d)That the terrestrial ancestor of the 

 sirenians had a sinuous body with long tail that was especially robust at 

 base, and was of a rather active disposition. These premises seem neces- 

 sary in order that it could swim with the sinous movements in the ver- 

 tical plane that the otter now employs in swimming. But we surely 

 have no justification for considering that any member of the probosci- 

 dean stock was ever this sort of mammal, (b) The other alternative is 

 that the sirenian tail developed along the lines that it followed (verti- 

 cal flattening) for the same reason that the tails of the platypus and 

 beaver did. Of these I regard the platypus' tail as the more significant, 

 for this animal and the sirenians (at least the manati) utilize the tail 

 extensively for keeping near the bottom. Murie's illustrations, repro- 

 duced in figure 7, were drawn from life and show this in an interesting 

 manner. 



The subject of the sirenian tail should not be abandoned without call- 

 ing attention to another possibility. As with most highly specialized 

 aquatic mammals it is likely that the more distinct genera are of great 

 antiquity. It is very likely that the manatis on the one hand, and the 

 dugongs (and possibly Hydrodamalis) on the other began to diverge as 

 separate groups soon after the sirenian ancestor took to the water, if not 

 before. It is not improbable that this took place before the tail had ex- 

 perienced any aquatic modification at all, in which event the tail of the 

 manati and of the dugong has each followed, an entirely independent 

 course of development, diverging in details to a greater or lesser extent 

 according to varying conditions which each has experienced. Presum- 

 ably the tail of the dugong is of the higher aquatic type, but this does 

 not necessarily mean that it ever passed through the exact stage which 

 is now exemplified by the manati. Each type of tail may now be per- 

 fectly fitted to the needs of the animal and the manati may never de- 

 velop the more specialized tail of the dugong. There is the possibility 

 that the flukes of the latter have developed from skin folds of rather 

 limited extent near the tail tip, while the tail of the manati may have 

 been evolved from an appendage having lateral keels along its entire 

 length. There is little evidence either for or against this possibility. 



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