428 ALLIS. [VoL. XIV. 
of an already formed plate of connective-tissue origin. Schmid- 
Monnard (No. 38, pp. 109, 116) says, that, while that part of 
the squamosal of Salmo and Esox that immediately surrounds 
the lateral canal by which the bone is traversed is of osteo- 
blastic origin, other parts of the bone are of purely membranous 
origin, arise independently of the part enclosing the canal, and 
then fuse with it. In Esox, that part of the squamosal that 
forms the articular facet for the hyomandibular is even said by 
him to acquire, secondarily, a cartilaginous character, and hence 
to represent a third, histologically different if not independent, 
component of the bone. McMurrich (No. 23, p. 297), before 
Schmid-Monnard, described the first two of these three compo- 
nents as primarily separate and independent parts in several of 
the canal bones of larvae of Amiurus, and I find the same con- 
ditions in larvae of Amia. The frontal, for example, in 20 mm. 
larvae of Amia, consists of two wholly separate parts, a semicy- 
lindrical gutter lying immediately beneath the supraorbital canal, 
and a flat, plate-like portion lying mesial to the canal, directly 
upon the cartilage of the chondrocranium. Still further evidence 
of the existence and primary independence of two components 
in these bones seems to be found in Sagemehl’s statement (No. 
36, p. 38) that in certain teleosts the lateral canals are found 
enclosed in bony tubes, which lie wholly superficial to, and inde- 
pendent of the bones that underlie them. This condition, 
which Sagemehl considers as secondary, is thus, probably, sim- 
ply a persistence of the primary, independent condition of the 
two components of the bones. Rose (No. 34, p. 661) considers 
the observations on which Klaatsch bases his conclusions as 
wholly erroneous ; and Gaupp (No. 17, p. 80) thinks that they 
need confirmation. 
In Amia, and hence, doubtless, in all fishes, those particular 
parts of each canal bone that lie immediately beneath the sense 
organs to which they are related begin to develop while those 
sense organs are still exposed on the outer surface of the head 
of the fish. It is, therefore, certain that the development of the 
bone does not depend, genetically, in any way upon the ulti- 
mate formation of a canal. Bones that are strictly homologous 
may, accordingly, be found in certain fishes related to sense 
