AQUATIC MAMMALS 



doubt. On the whole it is difficult to judge in which of the three sirenian 

 genera the posterior border of the nares Hes farthest caudad, as the con- 

 formation of the details of this region differs considerably, but it seems 

 to be most pronounced in Halkore. We know that in bodily form 

 Hydrodajnalis and Halicore are more perfected for aquatic locomotion 

 than Trichechus, and from the skull it seems that Halkore is slightly 

 more modified aquatically than Hydrodamalis. In all three the orbit 

 •proper is comparatively small, following reduction in the size of the eye- 

 ball. The temporal fossae indicate that these muscles are quite well devel- 

 oped. Deserving of passing mention is the peculiar hypertrophy in 

 Trichechus of the zygomatic processes of the squamosals. 



Among the Cetacea the skull of the sperm whale departs most widely 

 from that of the usual terrestrial type than that of any mammal living. 

 But in some respects it occupies an intermediate position between that 

 of the mysticetes and of other toothed whales. A better idea of the 

 cetacean skull can be obtained from the accompanying illustrations than 

 from a description and only some of the points of most importance in 

 the present connection will be discussed. 



On the whole the bones of Cetacea are very spongy, and oily, and 

 this is more marked in the Mysticeti. Perhaps the latter circumstances 

 is in compensation for the fact that this group is not so prone to develop 

 particular fatty areas upon the head, nor cavities and ducts for the 

 accommodation of oil between certain muscles and beneath the blubber 

 layer. Winge (1921) has stated that "the effect of water pressure is 

 to develop unusual strength in those rostral bones which project farthest 

 forward", and many others have been of the same opinion. I cannot 

 subscribe to this belief. Of course the bones are heavy, as they must be 

 in large mammals, but I cannot see that they are relatively as heavy, 

 or rather that they form as strong a complex, as in land mammals. 



The rostrum of the usual land mammal is affected by several strong, 

 fundamental stimuli ; as a support for a nose which must be well de- 

 veloped as an apparatus for taking in air and odors, as a framework for 

 mystaceal pads with vibrissae that may be highly tactile and for the an- 

 terior facial musculature, as a housing for intricate turbinal bones, and as 

 a support for dental armature. In addition certain mechanical elements 

 are introduced following the fact that the lower rostrum supports the 

 arch of the nasal bones covering the nasal passages. The Cetacea are 

 the only mammals in which none of these stimuli is present, save very 

 simple and usually weak teeth. Small wonder that in this order the 

 rostrum departs widely in form from the average mammalian type. 



[112] 



