NO. 2 A STUDY OF MENISCOTHERIUM — GAZIN 33 



broadly into the foramen lacerum medium, and where the internal 

 carotid would enter the cranial cavity. This groove is probably not 

 the inferior petrosal sinus for a vein, as posteriorly it does not seem 

 to join or even approach, as far as visible, the jugular or foramen 

 lacerum posterius. I suspect the above-mentioned grooves on the 

 ventrolateral surface of the petrosal may be for branches of a tym- 

 panic nerve plexus. 



In the absence ( ?) of an ossified bulla it may be further observed 

 and interpreted that the facial nerve emerged from the periotic at an 

 aperture in the roof of the tympanic chamber anterodorsal to the 

 fenestra ovalis, extending downward, then flexing backward lateral 

 to the fenestra ovalis into the sulcus facialis which also housed the 

 stapedial muscle dorsoexternal to the fenestra rotunda. At this point 

 the facial nerve was again directed downward and slightly backward 

 at the anterointernal root of the small mastoid process. Medial to 

 this point of departure there is a small pedicle, evidently the tympa- 

 nohyal, on a posteriorly directed crest or ridge at its union with the 

 posterior rim of the fenestra rotunda. This ridge separates the above 

 fossa for the facial nerve from the foramen lacerum posterius and 

 joins the petrous and mastoid portions of the periotic ventrally. 



Anterior to the aperture in the petrosal where the facial nerve 

 emerges there is a deep, anteroposteriorly elongate fossa the roof of 

 which covers the facial canal ventrally and which continues anteriorly 

 after a short hiatus, presumably for a branch of the stapedial artery, 

 with the broad groove in the alisphenoid for the eustachian tube. 

 This fossa would also house the tensor tympani muscle. Lateral to 

 this fossa and partially ventral to it there is a broad exposure of the 

 mastoid near, and occasionally adjacent to, the postglenoid foramen. 

 This exposure of the mastoid, anterodorsal to the position of the 

 audital tube must have been about where the anterior leg of the 

 tympanic annulus made contact. It is immediately behind the pos- 

 terior margin of the crest or pedicle formed of the squamosal and 

 alisphenoid suspected of supporting a bulla. 



The lateral wall of the mastoid, when the squamosal covering is 

 removed posterodorsal to the above exposure adjacent to the audital 

 tube, shows a deeply intrenched, nearly circular, and occasionally 

 branching path for the venous sinus terminating in the postglenoid 

 foramen. It is interesting to note that the lower arc of this sinus, 

 directly above the position of the external auditory meatus, appears 

 to be less curved in small M. chamense than in M. robustum, as 

 observed in two individuals of each. 



