AQUATIC MAMMALS 



Cranial conditions in the Mysticeti are more difficult to describe and 

 the reader should turn to the illustration of Balaenoptera (fig. 19)- It 

 is seen in this skull that although the premaxillae and maxillae are long 

 and of very specialized shape, they really do not extend farther backward, 

 using the position of the eye as a criterion, than in the normal mammal. 

 As in odontocetes it is the central elements of the skull — frontals and 

 parietals — that have suffered crowding, but in a different manner. In 

 both does this section appear to have been squeezed between the rostrum 

 and occipital, but whereas in the odontocetes the occipital seems to have 

 remained more or less stationary while the base of the rostrum has been 

 forced caudad, in mysticetes the appearance is that the rostrum main- 

 tained its position while ^the occipital did the pushing, over-riding the 

 parietals and overhanging the inferior part of the frontals, which are 

 expanded to form the floor of the temporal fossae in an extraordinary 

 manner. Beneath this is situated the eye. The relative weakness of the 

 rostrum should again be emphasized. Reference to the illustration (fig. 

 19) shows that envisioning the mysticete skull as though it were but 

 a few inches in length one would never suspect that it was anything more 

 than a purely edentulous mammal whose rostrum was not subjected to 

 many vicissitudes of strain — certainly not that it acted as a support for 

 an enormous armature of baleen (especially is this the case in the balaenid 

 whales). But like other parts of the animal the weight of the baleen 

 is largely counteracted by the flotation of the water and the lack of much 

 resistance applied to it, such as is experienced by the grinding of an upper 

 dental armature against a lower, save in a fore-and-aft direction by water 

 pressure, allows for the relatively moderate strength of the whole ros-' 

 trum. 



The nasal bones of mysticetes are much reduced but are otherwise 

 approximately normal. As usual in terrestrial mammals they roof over 

 the nasal cavities, which are more gently sloping, and not vertical as in 

 odontocetes. As in odontocetes the interparietal has been eliminated in 

 mysticete adults, although Ridewood (1922) found it in small fetal 

 balaenopterids, but not in those of Sibbaldi/s or Megaptera. It was also 

 present in the Balaenoptera borealis dissected by Schulte (1916). The 

 palatine is excluded from the wall of the nasal cavity. The skull is 

 symmetrical, although Kiikenthal, with characteristic persistence in his 

 theories, attempted to prove asymmetry. He did show fractional differ- 

 ences, naturally, for it is rarely that any mammal skull has the two sides 

 precisely alike, but these were not sufficient to be designated as asym- 

 metry. 



[118] 



