ENDOCRANIAL ANATOMY OF OREODON 281 



the same as in Specimen I. The ridges corresponding to the 

 internal carotid arteries are, however, larger and much more 

 clearly marked. Two thread-like and somewhat tortuous vas- 

 cular ridges are also evident on this cast, each beginning at the 

 posterior lacerated foramen in close association with the internal 

 carotid ridge and ramifying between this vessel and the midline 

 in the area immediately rostrad of the depression for the poste- 

 rior clinoid processes. These ridges evidently correspond to 

 small arterial vessels and may represent the posterior communi- 

 cating arteries, though in modern ruminants and suidae the ves- 

 sels bearing this name are most variable in their development and 

 are commonly not lodged in bony grooves. 



The shape of the pituitary protuberance also presents an inter- 

 esting variation in so far as in its rostral portion it is much broader 

 and more deep and shows evidence of bilateral symmetry, hav- 

 ing an ill defined median ridge over its most prominent part. 

 Between the pituitary prominence and the base of the chiasma 

 ridge is a well marked and somewhat bulging area which from 

 its surface configuration apparently marks the site of the infun- 

 dibulum. The caudal portion of the pituitary prominence is 

 depressed and the fossa occupied by the posterior clinoid processes 

 is somewhat deeper than in Specimen I. 



The broad caudal expansions of the pyriform lobes are each 

 marked by shallow longitudinal grooves. 



Immediately caudad of the posterior clinoid depression a 

 slight bulging of the smooth basioccipital surface apparently 

 indicates the site of the pons. This bulge is limited caudally by 

 an indistinct transverse groove in which are two minute but dis- 

 tinct depressions. Laterally this region is bounded by ridges 

 corresponding to the posterior lacerated foramen. The two emi- 

 nences corresponding to the precondylar foramina are evident 

 on the lateral aspects of the caudal part of the myelencephalic 

 base. 



The lateral surfaces of the brain stem are moulded in a char- 

 acteristic fashion by the periotic bones. These structures are 

 indicated by smooth depressed areas bounded on their lower mar- 

 gins by the posterior lacerated foramen and each marked in its 



