24 X^-S. NATIONAL MUSEUM BULLETIN 284 



a cupshaped articulating surface (fossa) for the dorsal anterior 

 condyle of its respective hyomandibular (the lower half of the articu- 

 lar surface for the hyomandibular is formed by the prootic). A 

 longitudinal ridge on each sphenotic external surface gives rise to a 

 ventrolateral process (the sphenotic spine) just dorsoposterior to 

 the level of the anterior hyomandibular condyle. 



Externally, most of the dorsal portion of each sphenotic is over- 

 lapped by the internal surface of the bladelike portion of its respective 

 pterotic, the ventral margin of which rests in a longitudinal sphenotic 

 groove that passes just internal to the sphenotic spine. The supra- 

 temporal canal joins the pterotic canal, which passes over the 

 sphenotic's anterolateral surface in the region overlapped by the 

 pterotic (the outer wall surrounding the canal is formed by the 

 pterotic). The extreme dorsoposterior extension of each sphenotic is 

 overlapped by its respective parietal. The slender anterior extension 

 of each sphenotic overlaps the dorsal external surface of its respective 

 pterosphenoid and is overlapped by its respective frontal. A ridge 

 on the internal surface of each sphenotic forms a synchondral joint 

 with the dorsoposterior margin of its respective pterosphenoid. At 

 its posterior end, each sphenotic forms a synchondral joint with its 

 respective pterotic; at its ventroposterior margin, a synchrondral 

 joint with its respective prootic. 



A broad, shallow groove on the internal posterior end of each 

 sphenotic is continuous with a similar groove on its respective parietal. 

 Part of the anterior vertical semicircular canal of the ear lies in this 

 groove. 



A narrow canal passes diagonally through each sphenotic. The 

 anterior opening of the canal is on the ventro-antero-internal margin 

 of the sphenotic's broad portion just dorsal to the articulating surface 

 for the hyomandibular. The posterior opening of the canal is in a 

 groove just mesial to the posterior margin of the sphenotic spine (just 

 external to the groove on the sphenotic inner surface for the tube of 

 the anterior vertical semicircular canal). Although the nerves were 

 not investigated, it is probable that the n. ramus oticus passes through 

 this canal. 



Pteeosphenoid (figs. 2, 4). — Ventrally, each thin, internally con- 

 cave pterosphenoid lies internal to the dorsoposterior portion of its 

 respective ascending wing of the parasphenoid and posteriorly internal 

 to the anterior portion of its respective sphenotic. The ventroposterior 

 margin of the pterosphenoid forms a synchondral joint with the 

 internal dorso-anterior margin of the prootic, while an anteriorly 

 extending projection of this prootic margin is external to the posterior 

 margin of the pterosphenoid. The dorsoposterior margin of each 



