AQUATIC MAMMALS 



mystacial pad with vibrissae, functional facial musculature in this region, 

 and the recession of the nares and narial musculature. We have no means 

 of knowing just what effect these alterations alone would have upon the 

 skull — what proportion of present conditions in Cetacea should be 

 ascribed to them and what to water resistance — for in no other mammal 

 have they all operated ; but it is evident that they would have some defi- 

 nite effect. At least this has been followed by some atrophy of the 

 rostral tip in vertical dimension, and it is not unreasonable to look for 

 a compensational hypertrophy in these elements at the rostral base. In 

 mysticetes this is found to some extent in the broadening of this part 

 of the rostrum. But in Patriocetus of the Oligocene both the widening 

 of the rostral base and the attenuation of its tip is more marked than in 

 any living mysticete, showing that both of these details have experienced 

 secondary modification through requirements of the baleen equipment as 

 this developed. In odontocetes this rostral change has taken the form 

 both of the broadening of the rostral base and the spreading of the 

 maxillae over the frontals. This as a lone stimulus would probably not 

 be of very great importance to telescoping, but it must be considered as 

 contributing in some degree to the total of stimuli for skull change. 



The odontocete skull is so formed that both the rostral tip and the 

 forehead receive the backward pressure of the water, while in ir^sticetes 

 the forehead does not receive it. The concomitant condition of the 

 telescoping of the rostral elements in the former group is not believed 

 to be purely fortuitous. At least it is presumed that the water pressure 

 at this critical point would be largely instrumental in strengthening the 

 anterior wall of the braincase. Admittedly this might be accomplished 

 as effectively by a mere thickening of the bones, but the fact remains that 

 the end has been attained by a process of laminations. The theoretical 

 effect in this connection of the presence of a frontal adipose cushion is 

 of course unknown, but it would naturally have some sort of stimulus, 

 probably in broadening the face. These stimuli are absent in mysticetes 

 and there has been no lamination of rostral elements over the frontals. 



In skulls of toothed wales (excepting present conditions in the Physe- 

 teridae and the Kogidae) the nasal passages have migrated just as far 

 to the rear as the anterior wall of the braincase has allowed. With them 

 has shifted a complicated muscular mechanism for the control of the 

 blowhole. This is no system of small muscular slips, weakly attached 

 to the bone, as in many mammals, but a robust complex, firmly anchored 

 to the skull over the entire breadth of the forehead and converging to 

 the blowhole and a part of the adipose cushion. These have dragged 

 [122} 



