AQUATIC MAMMALS 



In other words, such broad frontal prominences as occur have developed 

 not haphazardly but for a purpose that is of the utmost importance to 

 those species having them. 



Broadly speaking the shape of the cetacean head is of two sorts. In 

 the Odontoceti the throat contour passes almost straight back, while that 

 portion of the head above the rostrum offers by far the greater part of 

 the cephalic resistance to the water. In the Mysticeti the rule is for 

 the dorsal outline of the head to extend straight back from the rostrum 

 virtually parallel with the back, so that this part of the head does not 

 offer aquatic resistance, while such resistance as is offered is experienced 

 by the throat or the lateral dilation of the head. An exception to this 

 rule is found in the Balaenidae, with their rostra excessively curved a 

 phenomenon which is a more recent specialization. 



The rostrum of the toothed whales is exceedingly variable. It may 

 be so short that the fatty prominence of the forehead projects beyond it 

 (Globiocephala) or it may extend slenderly for almost four feet (Zar- 

 hachis) . Presumably those with no beak to speak of should feed on prey 

 that is not particularly agile, while a moderately projecting beak is of 

 advantage in snatching fish that move at considerable speed. But theo- 

 retically an excessively long beak such as that of the Miocene Zarhachis 

 would be more of a disadvantage than otherwise. With a mobile neck 

 a beak of this sort could be thrust quickly in all directions largely inde- 

 pendently of the position of the body, but in the Cetacea, with their 

 much shortened neck, the beak would have but slight directional mo- 

 bility, so that to effect a decided alteration in the direction of the beak 

 thrust the whole body would have to be moved. Evidently the ances- 

 tor of these long-beaked porpoises responded first moderately to some 

 stimulus for a lengthening of the rostrum but it seems very likely that 

 this modification attained undue evolutional velocity, developed first be- 

 yond the needs of the animals and then became of positive disadvan- 

 tage, so that it seems likely that the handicap of such an excessively long 

 rostrum may have contributed materially to their final extinction. 



The external form of the rostrum in the Mysticeti at least as we now 

 find it is probably largely attributable to mechanical needs. We can 

 tell little about it save that presumably the rostrum has developed in a 

 way most suitable to act as a support for the baleen and in response to 

 the need for a large mouth. The external grooving upon the throat 

 and chest of some of the whalebone whales has evidently been brought 

 about in the same way, and will be discussed later. 



[56] 



