18 



PROCEEDINGS OF THE NATIONAL MUSEUM 



VOL. 73 



length of the crest, while in Phoca hispida they are segregated in 

 the regions of the mastoid process and the medial part of the 

 occipital crest. And the latter is therefore the part that has been 

 pushed rostrad in response to certain muscular stimuli, as will be 

 discussed elsewhere. The part of the skull anterior to the supra- 

 occipital has resisted in varying degree the rostral push of the latter 

 bone, the force of resistance being supplied by the density of the 

 water through which the pinniped moves during locomotion. The 

 result, where marked, may be compared to the j&rst stages of the 

 *' telescoping " of certain of the cranial elements, as exemplified to 



MASS, 5UPtRF\C. 



Pig. 6. — Lateral view op the left mandible of Zalophus (Z) and of Phoca 

 hispida (p), sh0v7ing aeeas of mdscle attachments 



such a remarkable degree by the cetacean skull. An indeterminate, 

 though probably small, number of fissiped Carnivora exhibit to 

 some slight extent a sliding movement of the occipital plane, a pre- 

 requisite being a squamous, rather than a dentate, type of suture 

 between the bones involved, and some mechanical stimulus is undoubt- 

 edly also a necessity. Thus in the skull of an immature Arctonyx 

 leucolaemMS ohscurus (which is supposed to use its nose in digging) 

 with a length of 123 mm. the supraoccipital overlaps the parietals 

 by as much as 8 mm. In the immature skull of an otter {Aorvyas 



