SKULLS OP THE ZKUGLODONTlD^. 



651 



specimen Mii.l. It is remarkable for its extreme narrowness 

 compared with its height, the lateral portions of the Jambdoidal 

 crests being sliarply deflected backwards so that the deeply con- 

 cave occipital surface has the appearance of being pinclied in at 

 the sides, "^riiis character is also Avell shown in the cast of the 

 second skull (M. 8150). If the height be taken as 100 the width 

 is about 44. Tlie occipital condyles and foramen magnum are 



Text-figure 24. 



ismbdo'tf rrest 



process of 



Parocc/pital process. 



Occ'p'te/ contiy/e. 



Occipital surface of skull of Zeuglodon osiris Dames. M. 10328. 

 About 4 nat. size. 



still smaller than in Zeuglodon intermedms and the convex 

 sagittal crest is much higher. Tlie side walls of the skull show 

 little traces of convexity. The snout seems to have been rela- 

 tively narrower and perhaps the nostril opened rather further 

 back than in Zeuglodon intermsdius. 



The changes occurring in the stages described are : — 



(1) Increasing narrowness and concavity of the occipital 

 surface above the foramen magnum, due mainly to the 

 folding back of the high lambdoidal crests ; 



(2j increased height of the sagittal crest; 



(3) the reduction in the size in the condyles and foramen 



magnum ; 



(4) decreasing convexity of the lateral cranial walls accom- 



panying the depression of the upper portion of the 

 brain ; 



(5) the increase in length and slenderness of the snout and 



the probable recession, to some extent, of the narial 

 opening. 



Changes (1) and (2) result in the increase of the surface for 

 the attachment of the jaw-muscles, and are no doubt correlated 

 with the increasing size and strength of the mandible. 



In Frozeuylodon the two posterior upper premolars have a large 



