ENDOCRANIAL ANATOMY OF OREODON 279 



to skull grooves for the lodgement of the middle cerebral arteries. 

 On the caudo-lateral surface of the pyriform lobe a large obliquely 

 placed ridge indicates the site of a venous sinus (here termed the 

 infero-lateral sinus) which drains backward into the posterior 

 lacerated foramen and communicates with the parieto-temporal 

 channel. 



Above the anterior rhinal fissure a small triangular depressed 

 area is present which was termed the trigonum Sylvii in my pre- 

 liminary report (1. c). At that time the significance of the unu- 

 sually rostral situation of this area was not clearly understood. 

 However, subsequent examination of the corresponding region 

 in the other casts, especially in Specimen V, leaves but little room 

 for doubt as to the true homologies of the parts in question (vide 

 infra) . 



The trigonum Sylvii is bounded in front by the presylvian sulcus 

 which rostrally is bent sharply downwards towards the anterior 

 rhinal fissure. This is due to the overlapping of the presylvian 

 area by the prominent rostro-lateral gyri, in front of which the 

 presylvian sulcus may again be identified. 



Dorso-caudally the trigonum is limited by two opercular masses 

 bounded respectively by the anterior and posterior ectosylvian 

 sulci. At the junction of these sulci a small but distinct depres- 

 sion is evident. Above this depression and separated therefrom 

 by but a slight interval is a small oblique sulcus which has been 

 identified as the processus acuminis ectosylvii. 



The suprasylvian sulcus is well marked and at its caudal end 

 shows a slight bifurcation. Rostrally the suprasylvian sulcus 

 does not appear to be joined to the corono-ansate sulcus, in which 

 relation it differs from the condition obtaining at the left side of 

 this cast and on both hemispheres of all the other specimens 

 examined. A slight downwardly directed notch is evident on 

 the course of this sulcus at a level somewhat rostrad of the pro- 

 cessus acuminis ectosylvii. A short distance caudad of the latter 

 sulcus and parallel to it, a sulcus is present which from its rela- 

 tions has been termed the ramus descendens suprasylvii. 



The lateral sulcus forms a deep and well marked groove on the 

 dorsal aspect of the hemisphere. It is not connected with the 

 corono-ansate sulcus. 



