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end of that thin median ridge-like portion of the basioccipital that separates the saccular grooves, 

 and that supports, on its dorsal edge, the mesial edges of the mesial processes of the exoccipitals. 

 There, the mid-longitudinal line of the floor changes abruptly in level, the immediately posterior 

 portion lying at a slightly higher level, in a nearly horizontal position, on the dorsal surface of the 

 platform formed by the united mesial processes of the exoccipitals. Posterior to this platform the 

 floor slopes into the cavum sinus imparis, posterior to which there only remains the narrow edge 

 of bone that forms the ventral bounding edge of the foramen magnnm. 



The thin raised platform formed by the united mesial processes of the exoccipitals lies on 

 a level witb the pituitary opening of the brain case, and under this platform, on either side, lies the 

 posterior portion of the large saccular groove. Anterior to the exoccipital platform the saccular 

 grooves diverge, on either side, the central portion of the floor of the cavity widening gradually 

 from a thin median line at the edge of the platform, to its widest portion, immediately anterior to 

 the anterior ends of the saccular grooves. At either lateral corner of this widest portion lies the tri- 

 gemino-facialis recess, in which are the facialis, trigeminus, palatinus and profundus foramina, and 

 approximately between these recesses, in the median line, is the pituitary opening of the brain case. 

 Between the recess of either side and the anterior end of the corresponding saccular groove, there 

 is a thin bony partition which forms the ventro-mesial portion of the anterior wall of the large 

 labyrinth recess, that wall extending from there antero-dorso-laterally across the proötic and 

 sphenotic. In the lateral wall of the labyrinth recess there are four small recesses, all related to the 

 semicircular canals, and all separated from each other, and more or less surrounded by important 

 cartilaginous remnants of the chondrocranium. In the bottom of the anterior one of these four 

 recesses there are two depressions, separated by a low rounded ridge. The ventro-mesial depression 

 lies in the proötic and lodges the ampulla of the anterior semicircular canal, the dorso-lateral 

 depression lying in the sphenotic and being related to the rounded antero-dorsal corner of the 

 anterior semicircular canal, that canal lying wholly exposed in the cranial cavity. The next posterior 

 one of the four recesses also has two portions, one of which lies in the proötic and lodges the ampulla 

 of the external semicircular canal, while the other portion leads into the pterotic and encloses the 

 lateral portion of the same canal. The postero-ventral recess has three portions. a little pit-like 

 depression in the exoccipital, to lodge the ampulla of the posterior semicircular canal, and the 

 cerebral openings of two canals which enclose respectively the hind end of the external canal and 

 the ventral end of the posterior canal. The dorso-posterior recess lies in the epiotic, is large, and 

 has, in its ventro-posterior wall, a small opening which leads into the canal for the posterior semi- 

 circular canal, that canal traversing the epiotic. In Amia, the hind wall of the labyrinth recess is 

 formed by an important cartilaginous ridge, membranous in its middle portion, which projects antero- 

 mesially from the lateral cranial wall and separates the labyrinth recess from an important posterior 

 portion of the cranial cavity which I described ('97, p. 703) as the postauditory or occipital Chamber. 

 If the membranous ear of Amia, and in particular the sacculus, were to be greatly developed, the 

 labyrinth recess would have to be correspondingly enlarged, and this would necessarily push the 

 posterior wall of the recess backward and mesially, the vagus foramen remaining always posterior 

 to the wall. As the saccular recess was thus pushed backward it would split the dorsal edge of the 

 basioccipital and the ventral edge of the exoccipital each into two parts, one of these parts forming 

 the outer and the other the inner wall of the recess, and so give rise to a saccular groove on the 

 dorsal surface of the former bone and to a mesial process, roofing that groove, on the internal sur- 



