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 sphenotie 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’ 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’ also points out that the sphenotic postfrontal) and also 
the prefrontal (ectethmoid) in Ama 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 orbitosphenoids 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 notochord, surrounded 
with some osseous tissue apparently developed from its sheath, as in 
the vertebre. The lower angles of this ossification are continuous 
1 Schmid-Monnaid.—Die Histogenese des Knochens der Teleostier. Zeit. f. wissZool Bd. 
XXXIX., 1883. 
2Loc. cit. 
