AQUATIC MAMMALS 



middle or posterior ones, they are quite a bit thicker than the last cervical. 

 This may be explained, I believe, by the probability that the posterior 

 cervicals have become as thin as possible without fusion, and thinner 

 than any rib-bearing vertebra could become. 



Not only is the skull relatively larger in all whales than in the seal, 

 but almost invariably that of the former has accessory equipment in the 

 way of special fatty deposits or baleen armature, which makes the head 

 still larger in comparison to body mass. The head alone therefore fur- 

 nishes almost or quite the prethoracic mass and weight requisite to high 

 swimming efficiency. In other words the head alone is so large that if 

 there were a neck of respectable length the part of the animal anterior 

 to the thoracic "pivot" of motion in swimming would either be too mas- 

 sive for the part posterior thereto, or else, if the pivot could be shifted 

 forward, and therefore from the thorax to the skull, the neck would 

 really constitute a mechanical part of the body. 



In scrutinizing various cranial and cervical conditions there seem to be 

 recognizable two possible factors which might show a tendency for re- 

 tarding maximum shortening of the neck and fusion of its elements. One 

 of these is the condition imposed by the presence of a tweezer-like beak, 

 or of the comparable tusk of the narwhal. It is readily seen that this 

 equipment would necessitate at least moderate shifting about of the head 

 with a consequent amount of flexibility of the neck, and this should delay 

 fusion of the vertebrae. At any rate the narwhal has all the cervical ver- 

 tebrae free, as have such long-beaked forms as Platanista and the extinct 

 Eurhinodelphis. The other is the fact that in those forms with long or 

 slender beaks the cranial part of the skull is relatively small in relation to 

 body size. Hence the head has less mass and this might be compensated 

 for by a slightly longer neck. But non-beaked odontocetes such as the 

 beluga (Delphwapterus) may also have the cervicals free, and this fact 

 introduces an element of serious doubt. 



At the other extreme are such odontocetes with head of moderate ceta- 

 cean size as Hyperoodon, Ziphius, Mesoplodon. Grampus, and at times 

 Fseudorca and Phocaena, in which all the cervicals are fused into one 

 solid bone. Still other sorts exhibit every intermediate condition between 

 complete fusion and complete freedom of the cervical elements, but in 

 all existing Delphinidae excepting the narwhal and beluga at least the 

 atlas and axis are fused. At one time I endeavored to reach some con- 

 clusion regarding this point by making a list of cervical conditions in all 

 species of whales which I could examine, and attempting to correlate 

 them with any one of numerous other osteological features ; but with re- 

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