498 ALLIS, [VoL. II. 
much smaller than the openings of the other pores, leads into 
the infra-orbital canal, which at this point is deflected toward 
the lateral edge of the squamosal. In the first division of this 
double system the trunk disappears, and the two primary 
branches travel downward along the opercular canal, at the 
‘same time separating somewhat along the line of that canal. 
The two secondary pores formed by this division also separate 
antero-posteriorly. From one of them arises the half-system 
lying in the adult in front of the canal (Fig. 40), and from the 
other that lying behind it. 
Trunks 18 and Ig enter the main canal on either side of the 
extrascapula, between it and the squamosal and suprascapula 
respectively ; No. 20 enters it between the suprascapula and 
supraclavicula ; No. 21 through the supraclavicula, and No. 22-1 
between the supraclavicula and the first scale of the lateral line. 
In the supra-orbital line trunks 1, 2, and 3 all enter the canal 
through the nasal; No. 1 on its lateral edge; No. 2 on the ante- 
rior edge; and No. 3 through the top of the bone. Trunk 4 
enters the canal between the nasal and frontal; Nos. 5 and 6 
through the frontal; No. 7, as already described, between the 
frontal and postfrontal; and No. 8 between the frontal and 
parietal. 
In the operculo-mandibular line, trunks 1 to 7 enter the canal 
through the dentary; No. 8 between the dentary and nasal; 
Nos. 9 and 10 through the angular; No. 11, a compound or 
double trunk, between the angular and pre-operculum; Nos. 
12 to 16 through the pre-operculum ; and No. 17 between the 
upper end of this bone and the lateral edge of the squamosal. 
In the supratemporal cross-commissure the two lateral trunks 
on each side enter the canal through the extrascapulz, and the 
large double median one between the two bones. By the first 
division of this median pore the two primary branches are 
formed, which, in the adult, become the trunks of the half- 
systems lying one on each side of the middle line of the head. 
The conditions existing at this age and a little earlier show that 
a primary tube originally leaves each of the main lateral canals 
along its entire length between cach two consecutive dermal bones, 
and that at every point of union of two canals, whether of the 
same or of opposite sides of the head, a double pore and trunk ts 
formed, which give rise at this point to a double dendritic system. 
