26 U.S. NATIONAL MUSEUM BULLETIN 2S4 



through these foramina. At the external antero ventral point of junc- 

 tion of each prootic with the parasphenoid is an opening (carotid 

 foramen) to an anteriorly extending canal, the walls of which are 

 formed by the parasphenoid and prootic. 



Internally each prootic is divided by a thin arching ridge into a 

 large anterior and a small posterior section. Ventrally each section 

 is concave. Dorsally the anterior section is a wall pierced by the two 

 openings in the region of the bridge. The posterior upper section is 

 concave and separated by a narrow convexity from the lower con- 

 cavity. The anterior ventral concavity sm*rounds a portion of the 

 brain. The posterior ventral concavity is actually half of a larger 

 concavity formed with the exoccipital (partly floored by the basi- 

 occipital) against which the sagitta lies. (The convex mesial surface 

 of the sagitta surrounded by the sacculus projects into the cranial 

 cavity and rests against, or close to, the brain.) The tiny lapillus is 

 situated just dorsal to the sagitta in the ventroposterior portion of 

 the posterodorsal prootic concavity. (The asteriscus is just posterior 

 to the sagitta and rests against the wall of the exoccipital.) The 

 otoliths, in their membranous sacs, project into the cranial cavity 

 and are not separated by bone or cartilage from the brain. The otoliths 

 are illustrated in figure 6. The membranous tube of the anterior semi- 

 circular canal lies along the posterior surface of the arching ridge, 

 which divides the prootic into anterior and posterior portions. This 

 ridge continues dorsally beyond the prootic and becomes a concavity 

 on the internal surfaces of the sphenotic and parietal, conforming to 

 the membranous tube of the anterior semicircular canal. 



Epiotic (figs. 2-5). — Each epiotic is a domeshaped bone with a 

 raised, large, flattened, external process that articulates with the ven- 

 tral surface of the dorsal arm of its respective supratemporal. The 

 anterior end of this process lies closely adjacent to the parietal crest 

 and forms a roof over the mesial arm of the "1 -shaped lateral extra- 

 scapular. The process indents anteromedially and forms the posterior 

 border of a dorsal opening to the supratemporal laterosensory canal (the 

 anterior border formed by the parietal crest). The internal perimeter 

 of each epiotic dome, similarly shaped to that of the pterotic internal 

 perimeter, is ringlike and forms synchondral joints with its respective 

 exoccipital ventroposteriorly, pterotic anteriorly, and the supraoc- 

 cipital dorsoposteriorly. Anteriorly the external epiotic surface con- 

 tacts its respective parietal's ventropostero-internal margin. An 

 arched groove on the internal epiotic surface continues as a tube 

 through the bone through which passes the posterior vertical semi- 

 circular canal of the ear. 



Externally each epiotic is posteriorly overlapped by a portion of 

 the supraoccipital. 



