AQUATIC MAMMALS 



terior direction. Hence the force of the water pushing against the tip 

 is distributed over the anterior part of the skull segment next caudad. 

 Even in the baleen whales is this largely true, for when the mouth 

 is open and the baleen is subjected to water force the animal is presum- 

 ably always swimming at low speed. 



If we examine the stimuli experienced by other parts of the head we 

 find that the eyes are reduced and unusually placed, the function of the 

 ear has altered, the masticatory musculature is reduced in odontocetes 

 and altered in all, the cranium proper must, with the base of the rostrum, 

 act as an anchor for the narial musculature, and the need for supporting 

 the head has been simplified by flotation. Hence almost all of these are 

 different from what the usual land mammal meets. 



In zeuglodonts the bones of the skull retain essentially the normal 

 mammalian relationship, although by a recession of the nasals the bony 

 nares have become elevated from one-half to two-thirds the distance to 

 the orbit (Kellogg, 1928). But in some other respects, as reduction of 

 the hind limb, these mammals at the time of their extinction were al- 

 most as highly aquatic as most existing whales. Hence it is evident that 

 they were either inherently lacking in the ability to respond to the stimuli 

 that have resulted in the telescoping of the cetacean skull, which, under 

 the circumstances, appears unlikely, or else that they did not experience 

 those stimuli that were most critical in bringing about telescoping. What 

 these may have been will be discussed in the case of existing Cetacea, 

 but it may here be mentioned that the mere fact that the gradual recession 

 in zeuglodonts of the anterior border of the nasals to a position from 

 one-half to two-thirds the distance to the orbits does not necessarily 

 mean that the external nostrils must have shifted to this position. This 

 has been accepted without question, it seems, but the bony nares lie far 

 to the rear of the nostrils in sirenians, and the same may have been true 

 in zeuglodonts. Not only that but the recession of the nasals may, as 

 in the moose, indicate that they had a small proboscis, and so the nostrils 

 may actually have been situated anterior to the tip of the rostrum. If 

 this were really the case, then the zeuglodent skull lacked all those 

 stimuli which I now believe to have been at all important in bringing 

 about the condition of telescoping, except that of backward water pres- 

 sure against the head. 



As Miller (19'23) says, in "modern Cetacea the most conspicuous 

 facts are these: (a) That the telescoping of the skull was far advanced 

 in the earliest known extinct genera, and (b) that this process has de- 

 veloped according to two different plans". Briefly, in odontocetes the 



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