No. 3.] LATERAL LINE OF AMIA. 497 
os mastotdeum of the adult Cottus. The gutters then become 
tubes, lying at first on the upper surface of the bony plates, and 
corresponding to the conditions found in the adult of some of 
the bony fishes. 
The relations of the primary tubes to the various dermal 
bones in Amia are easily determined in a series of sections. 
They differ slightly from the relations of the trunks of the 
corresponding systems to the bones in the adult. 
Trunk No. 2 infra-orbital enters its canal through the eth- 
moid; No. 3 between the ethmoid and the antorbital; Nos. 4 
and 5 through the antorbital, and No. 6 at its upper posterior 
edge; No. 7 between the antorbital and lachrymal; No. 8 
through the lachrymal; No. 9 between the lachrymal and first 
suborbital; No. 10 between the first and second suborbitals ; 
No. 11 between the second suborbital and lower postorbital ; 
No. 12 through the lower postorbital ; No. 13 between the post- 
orbitals ; No. 14 between the upper postorbital and the post- 
frontal. 
The double trunk 15-7 lies between the frontal and postfrontal 
and between the two anastomosing canals, which, at this point, 
change sharply their direction. The infra-orbital lies entirely 
in the postfrontal, and the supra-orbital entirely in the frontal ; 
both of them along the open edges of the bones. The trunk of 
the system at this age is a large shallow pit into which the two 
canals open. In the further development of the system it 
deepens, becomes narrower, and lies over the point of anastomo- 
sis of the two canals instead of between them as at first. In 
the first division of the system the trunk divides after the man- 
ner shown in Cut 3, p. 18, fand g. One of the primary branches 
resulting from this division retains its position at the point of 
anastomosis of the two canals. The other travels backward 
along the infra-orbital canal till it reaches the anterior edge of 
the squamosal, where it is found in the adult (Fig. 40). The 
trunk of the system disappears. 
Trunk 16 infra-orbital enters its canal through the squamosal. 
Immediately behind it is the double opening, 17-17, which lies 
at the lateral edge of the squamosal, between it and the upper 
end of the pre-operculum, and resembles closely the double pore 
15-7. The upper end of the opercular line opens directly into 
the pit on its lower wall, and on the upper wall a small opening, 
