AQUATIC MAMMALS 



spinalis capitis, rhomboideus anticus, humerotrapezius and cephalohu- 

 meral are confined to a narrow area near the vertex, while the sterno- 

 mastoid, trachelomastoid, and splenius are confined narrowly to the 

 mastoid process. Thus, in this animal, the most powerful muscles for 

 motivation of the dorsal region are confined to two circumscribed areas, 

 one for vertical movement of the occiput and the other for lateral move- 

 ment, which is an excellent indication that Phoca has very much less 

 need for such complex cranial actions as are indulged in by the sea-lions. 

 As already indicated, because of the fundamentally different positions 

 of the primary swimming apparatus in these two pinnipeds, the otariid 

 can move its head in all manner of ways without disturbing equilibrium, 

 or else this may be compensated for by movement of the hind feet ; and 

 furthermore, the head and neck, by swaying motions, are of decided use 

 in the terrestrial locomotion of this animal. On the other hand, the true 

 seals travel on land by vertical undulations of the body, in which the head 

 and neck do not play an important part, and because the swimming 

 organs are at the rear, the neck, and consequently the head, can be moved 

 only in moderate degree during swimming, as discussed in succeeding 

 chapters. 



The only cranial modifications of the order Sirenia which may with 

 certainty be ascribed to aquatic influences are the exceeding density of 

 their bones, the recession of the nares, probably, and the slight forward 

 tilting of the occipital plane, conforming to the usual posture of the head. 

 In addition, adults of the extinct Steller sea cow (Hydrodamalis) were 

 toothless, the place of the teeth being taken by horny oral ridges devel- 

 oped for masticating soft marine algae. The phenomenally long, de- 

 pressed snout and mandible of the dugong is doubtless for the same pur- 

 pose, as these support horny rugosities of the membrane which should be 

 equally efficient for this purpose. Why bones of the Sirenia should be 

 denser, harder and heavier than in other aquatic mammals is unknown, 

 but presumably they have become so in response to a long continued need. 



In Sirenia the recession of the bony nares is marked (fig. 16), and 

 although a lengthy description is hardly necessary it may be mentioned 

 that in this group the posterior narial border is well behind the orbits 

 proper, and that anteriorly there is a great narial basin between the pre- 

 maxillaries. But the external position of the nostrils is near the anterior 

 angle of the snout, to allow for breathing at the surface with no other 

 part of the head exposed, a position that is definitely forward of any part 

 of the narial basining of the skull. So it is seen that in this group the 

 recession of the bony nares has been far posterior to the position indi- 



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