280 PROCEEDINGS OF THE CANADIAN INSTITUTE. 



cartilage of those regions commences. As regards the former it must 

 be noted that there is apparently a portion (the thin ledge-like por- 

 tion overlapping the anterior portion of hyomandibular) which is 

 formed entirely by membrane. These two bones, then, are partly 

 formed perichondrally, and partly from bone originating in mem- 

 brane, and, accordingly, objections to the pterotic being considered 

 equivalent to the other otic bones on account of its possessing a mu- 

 cous canal, are groundless, since the pterotic and sphenotic are in 

 reality cartilage bones for the protection of semi-circular canals, the 

 union of the membrane bone being secondary, and probably for the 

 purpose of increasing the strength of the protective tube of the mu- 

 cous canal. Schmid-Monnard 1 has recently pointed out the part 

 played by the mucous canal in the formation of the pterotic, but does 

 not seem to have noticed it in the case of the sphenotic. 



Sagemehl 2 also points out that the sphenotic postfrontal) and also 

 the prefrontal (ectethmoid) in Amia possess a membranous element, 

 but does not recognize in the sphenotic that the membrane bone 

 really belongs to the mucous canal. As regards the ectethmoid in 

 Amiurus, it is truly perichondral, for the mucous canal which lies 

 above it does not unite with it, but is separated from it by connec- 

 tive tissue. 



As regards the other bones, the prootics, epiotics, alisphenoids, 

 and basisphenoid, are entirely perichondral in their formation ; the 

 supraoccipital is partly perichondral and partly formed from a super- 

 ficial plate of membrane bone, which unites with the subjacent 

 perichondral; the orbitosphenoid is mainly perichondral, but the 

 cartilaginous orbitosphenoid s do not meet in the middle line, but 

 leave a space at the base of the skull bridged over by membrane 

 continuous with the perichondrium, from which the median basal 

 portion of the bone is developed. The ex-occipital, too, is mainly 

 perichondral, the two ledges which roof in the sinus impar being, 

 however, membranous in their origin. 



" The basioccipital, however, presents several points for considera- 

 tion in its development. In the young stage above mentioned, at 

 the median line at the base of the skull is the notochoi-d, surrounded 

 with some osseous tissue apparently developed from its sheath, as in 

 the vertebra?. The lower angles of this ossification are continuous 



1 Schmid-Monnarcl — Die Histogenese des Knochens der Teleostier. Zeit. f. vvissZool Bd. 

 XXXIX., 1883. 

 Hmc. cit. 



