AQUATIC MAMMALS 



the rostrum. I doubt whether this is a definite hindrance. If the stimu- 

 lus in other directions were of sufficient strength at least the superior 

 part of the maxillary could extend farther to the rear. Nor do I consider 

 that the possible presence of a dentate type of suture between the maxillae 

 and the frontals of the mysticete ancestor would be a permanent inhibitor 

 of marked telescoping in this region, as contended by Miller. Sutural 

 changes cannot be instigated in the adult skull, in which the attachments 

 of all bones are secure, but begin in the younger animal whose sutures 

 are not securely locked; and the individual time of sutural closure is 

 notoriously variable. As far as present knowledge goes the cranial 

 sutures will change according to the needs of the animal and it seems 



Figure 18. Dorsal view of the skull of the cachalot or sperm whale, Physeter. 



unlikely that they could have been so conservative and tenacious of 

 type in the cetacean ancestors as to predetermine the whole course of 

 cranial development in the two groups. 



There is little or no sliding of one bone over the other in the posterior 

 elements (occipital, parietal and squamosal bones) of the odontocete 

 skull, and no details that may not be understood rather readily. In the 

 existing sperm whale the occipital plane is vertical, which gives the ap- 

 pearance that all force has been from before toward the rear, but this 

 is probably a secondary result, for in the Miocene Diaphorocetus, and 

 doubtless other early physeterids, the facial basin appears to have been 

 not quite so well defined and there was a marked forward inclination 

 [124] 



