THE SKULL 



be compared in this way for the stimuH involved were doubtless very 

 different. Observation tells us that at least the stimulus for occipital 

 tilting in modernized mysticetes has been one of the strongest experienced 

 by the skull for the reason that it is more pronounced than in any other 

 mammal. 



The difficulty experienced in explaining telescoping of the odontocete 

 facial elements because no other mammal exhibits an intermediate con- 

 dition in this respect is not so applicable in the case of the telescoping 

 shown by the mysticete occipital. In the first place the latter is of a 

 simpler, less astonishing sort, with the lamination of the bones not 

 marked. In fact it is just about what one would expect to see result from 

 an excessive foreward tipping of the occipital plane in any type of mam- 

 mal. Several other mammals show this to a modified degree, and several 

 exhibit incipient telescoping of the posterior cranial elements. Among 

 terrestrial forms occipital tilting is perhaps most marked in the European 

 blind rat {Spalax) , but in all rodents sufficiently specialized for a fossorial 

 life as to be blind or nearly blind (Spalacidae, Rhizomyidae and Bathyer- 

 gidae) this character is very pronounced, while in burrowing mammals 

 not quite so strongly modified (fossorial octodonts, murids and geomyds) 

 the occipital change is but moderate or slight. It is also a character of 

 some few ungulates. 



That part of telescoping which is exemplified by the broadening of 

 the surface of contact between two bones is shared by Spalax also, and 

 by most if not all of the more specialized fossorial rodents, this con- 

 sisting in a broadening of the squamate sutural contact of occipital with 

 parietal. In varying extent it is found also in the pig tribe, which roots 

 in the ground, in Arctonyx the hog badger, which does the same thing, 

 and in the fur seal {Callorhinus) , but not the hair seal (Pboca) . In all 

 of these as well as in the Cetacea the head frequently or almost continually 

 experiences a posteriorly directed force applied against the muzzle. Water 

 presses against the head of the aquatic sorts as they swim, and the re- 

 mainder push against the ground, rooting, tamping the earth of exca- 

 vated runways with the nose (Spalax) or using the incisors as a pick to 

 loosen the earth of their burrows after the manner of many fossorial 

 rodents. Mechanically it would be very easy for the forward tilting of 

 the occipital to bring about a broadening of occipital-parietal suture. 

 Alone it might not have this effect, but the condition may well have 

 been helped by the backward pressure against the rostrum experienced 

 by those mammals which exhibit this tilting to best advantage. That 

 forward tilting of the occipital is not a requisite of marked broadening 



[127] 



