AQUATIC MAMMALS 



type the rostrum is rather evenly rounded from side to side, while in 

 balaenopterids it is flattened and expanded, so that in the latter the 

 rostrum must be kept safely elevated to obviate the danger that when 

 moving at thirty miles an hour the rostrum might become sufficiently 

 depressed for the water to take hold of the broad dorsal plane and force 

 the head sharply downward, with results that might be highly uncom- 

 fortable. 



In addition to the fact that the occipital should slope more in mysticetes 

 than odontocetes because the former hold the head more elevated, there 

 is the thesis, explained in the next chapter, that because of the larger 

 head the pivot of motion for swimming, normally situated in the anterior 

 thorax, has been shifted somewhat farther forward in mysticetes, which 

 might very likely have a tendency to force the occipital musculature more 

 decidedly rostrad. There is the further theory that the larger, flatter head 

 of the balaenopterids requires a longer power arm for the occipital 

 muscles than the short neck alone can provide ; and the possible influence 

 for the same effect suggested by the fact that in the balaenopterid the 

 semispinalis capitis is the one developed for chief control in elevating 

 the head, while in at least some odontocetes the less potentially active 

 posterior rectus is fully as important and may be far more so. In addi- 

 tion the rounded braincase of most porpoises shows that there has been 

 an endocranial opposing force which the occipital has had less success 

 in overcoming than in the much larger mysticetes. Finally, facial mi- 

 gration toward the rear is not so pronounced in mysticetes, and this sug- 

 gests that in odontocetes this stimulus for facial recession has proved 

 too strong for the occipital tilt to overcome so readily, and that its for- 

 ward inclination has at least been retarded by this influence. That this 

 latter is not a mere fancy is suggested by the situation in the sperm whale. 

 Although fossils of primitive type {Diaphorocetus) show a marked oc- 

 cipital tilt, backward pressure supplied by the developing spermaceti or- 

 gan has proved the stronger stimulus and overcome it, until now the oc- 

 cipital plane of the living cachalot is practically vertical. 



The above are at least some of the reasons accounting for the fact 

 that in Cetacea the forward tilting of the occipital plane is extreme, that 

 it is more pronounced in mysticetes than odontocetes, and more marked, 

 in relation to body axis, in balaenopterid than in balaenid whales. 



There are many having their own particular theories who will not 

 agree with me, but to me the evidence as marshalled above is sufficiently 

 strong for the acceptance, at least until many more data are available, 

 of the following thesis. That flotation and water pressure has had an 



[136} 



