Chapter Six 



; The Skull 



SEVERAL volumes might be written descriptive of the skulls of the 

 Cetacea alone, and indeed the literature on this subject is already so 

 voluminous that it would be unjustifiable here to do more than mention 

 the most salient points and discuss those details which are believed to 

 have particular bearing on aquatic modifications. 



Skulls of the Monotremata are so unique that there are but a few 

 details in that of the platypus that we can be sure are largely the result 

 of its aquatic habits. These, briefly, are the dorsal position of the orbits, 

 and the form of the broad, bifurcated rostrum and mandible as the 

 framework of the spatulate bill. No insectivore has any well marked 

 cranial modifications for an aquatic life, and almost the same can be 

 said for rodents. In this order it is popularly presumed that there is 

 some tendency for aquatic sorts to exhibit shortening of the nasals, to 

 allow for a more dorsal position of the nostrils, and flattening of the 

 dorsal side of the skull to permit of more dorsal vision ; but these char- 

 acters should be evaluated with caution. Evidently a straight dorsum 

 means little in itself. This part of the skull in such an essentially aquatic 

 genus as the muskrat (Ondatra) is indubitably convex, but is straight in 

 Hydromys. In the terrestrial wood rat subgenus Teonoma the inter- 

 orbital region is equally straight or gently concave, however. In Nilop- 

 egamys it is also concave, and still more so in Ichthyomys, so it is prob- 

 ably justifiable to accept this tentatively as an aquatic character in rodents. 

 In Hippopotamus the nasals are somewhat shortened and there is marked 

 elevation of the bony eye sockets. The peculiarities of the skull of the 

 tapir probably have nothing to do with its slightly aquatic habits. 



The skulls of aquatic fissipeds and of pinnipeds show only slight 

 aquatic adaptations in the details where one might expect them to be 

 most marked. They have rostra that are perhaps shorter than in the 

 majority of terrestrial fissipeds; but so have the cats and others. The 

 same may be said about the recession of the anterior margin of the nasals. 

 This is more pronounced in Phoca than Zalophus, but in relation to 

 total length of skull it is no more marked than in the Canidae, for in- 

 stance. The lachrymal bone has virtually disappeared in the pinnipeds, 



[107] 



