BRAIN OF THE ZEUGLODONTIDiE. 



631 



disappearing laterally below the paraflocculi. Here it separates 

 the paraflocculus above from the structures of the jugular leash 

 below. It is clue to a kind of secondary tentorial bony projection 

 on the interior aspect of the occipital bone. The presence of this 

 bony strut is perhaps to be associated with the increasing width 

 of the occipital region. This furrow is wider and more dorso- 

 ventrally situated in the cast of Prozei(,glodon atro.v ; whereas in 

 Zeuglodon osiris (vide infra) ifc is still transverse but very wide 

 and scalloped out, showing tliat with the extreme width of the 

 skull of Zeurjlodon osiris the bone has become greatly reinforced 

 in thickness internally. It is well to note such cases as these 



Text-figure 14. 



Lotus mecf/us cerebeJIi. 



Paraf/occu/us. 



Posterior view of 6ndocranial cast of Zeuglodon intermedius, sp, nov. 

 About i nat. size. 



M. 10173. 



where the modelling of the skull undoubtedly influences the 

 shape of the endocranial contents, because it shows that the inter- 

 play of factors (^. e., brain upon skull and skull upon brain) in 

 cavity-modelling is ceaseless and reciprocal. 



Coincident with the lateral expansion of the brain-substance 

 there has been no widening of the medulla oblongata or the 

 aporburo of the foramen magnum [vide Tables), but the thickness 

 of the medulla oblongata lias decreased somewhat and so ifc 

 presents a somewhat more ellipsoidal transverse section. 



The cubical capacities of these two casts was 790 c.c. for 

 Prozeur/lodon atrox and 785 c.c, for Zeuylodon intermedius. As 

 the cast of Prozeuglodon atrox v/as very incomplete anteriorly it 

 is evident that its cubic bulk must have been considerably over 

 800 c.c. 



The degree of likeness between these two forms both cranially 

 and endocranially is so great that they may be regarded as very 

 closely related phyletically. Because of this the decreased volume 

 of the brain of Zeuglodon intermediums, in accordance with the 

 "law of increasing brain weight" expressed by Marsh, indicates 



