38 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 76 



parietals of the rodent." The fact that the modification has gone 

 farther in whales than in rodents may perhaps be not entirely unre- 

 lated to the circumstances that the muscular activity in the one 

 instance is needed during the act of burrowing only, while in the 

 other it is needed at its maximum whenever the animal feeds ; and 

 the part of it which is required to oppose gravitation must be con- 

 tinuous from birth to death. An additional reason to believe that 

 the forward extension of the frontal shield in the baleen whales may 

 be in part related to the great development and peculiar function of 

 the rostrum and jaws is furnished by the fact that this extension 

 is in general more pronounced throughout the mysticete group than 

 it is in the toothed cetacea. Among the latter there is no enlarging 

 of the mouth to engulf great quantities of water ; the combined weight 

 of the rostrum and jaws is almost without exception much less, rela- 

 tively to the weight of the cranium, than it is in the mysticetes ; and 

 in no dolphin or toothed whale, no matter what the degree or kind of 

 telescoping, even in such excessively compressed types as Hypcroodon 

 and Kogia (pi. 7), does the occipital shield increase its area by push- 

 ing obliquely forward noticeably beyond the level of the articular 

 region, a level which it normally passes in all of the more specialized 

 baleen whales. (Compare Stenodelphis, pi. 7, fig. 2, a dolphin with 

 greatly elongated rostrum, and Eubalcvna, pi. 6, fig. 4, a member of the 

 other group ; in the former the anterior border of the occipital lies 

 slightly behind the articular level while in the latter it lies far in 

 front ; the articular area in each is situated below the reference 

 letters sq, and the position of the anterior edge of the occipital is 

 marked by the sign -1-.) 



The probability that water pressure has been one of the main 

 factors in determining the behavior of the modern cetacean skull 

 seems to be much strengthened by an examination of the characters 

 of the two other groups of mammals whose members have assumed 

 an exclusively aquatic mode of existence. These groups are the 

 zeuglodonts and the sea-cows. In the former the skull has retained 

 most of its generalized mammalian structure, in the latter it has 



' A definite overlapping of the hinder border of the parietal by the occipital 

 occurs in the pigs and peccaries, both of which use the snout for " rooting " 

 in an upward direction ; but the process in these animals differs from that 

 which is seen in the baleen whales and Spalax in that it takes place in a nearly- 

 vertical direction so that the occipital shield faces backward on the occipital 

 aspect of the skull. It appears to be associated with a pushing forward of the 

 base of the braincase which would indicate the action of some remodeling force 

 which does not operate in the whales and rodents. 



