THE SENSORY CANAL SYSTEM OF FISHES. 523 
of the canal there are a series of these bones. The canal is 
conducted along the sides of the body by alternating series of 
these small bones and of isolated scutes, into which latter the 
canal enters posteriorly and on the ventral side, passing out 
anteriorly and on the dorsal side. The number of these small 
drain-pipe-like bones between each scute varies from four to 
ten. The greater portion of the canal is exceedingly small in 
diameter, increasing slightly in the anterior portion. 
There are no branches given off from the lateral canal, 
neither in or on the scutes or from the intervening portions. 
There is also a complete absence of cluster or primitive pores. 
2. The Main Canal of the Head.—The main canal 
enters the skull by traversing the lateral border of the post- 
temporal. It is continued forwards through the epiotic, in the 
anterior portion of which it gives off the occipital commissure. 
Entering the squamosal the canal passes forwards to a central 
position and divides into two branches, the supra- and sub- 
orbital. 
The Supra-orbital Branch.—Leaving the main canal in 
the squamosal, the supra-orbital branch takes an inward and 
forward direction; passing through the frontal bone, in 
which it gives off two lateral branches, it takes an out- 
ward course between the nasal apertures and is continued 
on the dorso-lateral border of the rostrum, again passing 
into a series of drain-pipe-like bones. In the most anterior 
portion it makes a ventral turn and joins with the sub-orbital 
branch by passing through a small tooth-shaped bone (PI. 39, 
fig. 7, a and 0). 
The Sub-orbital Branch.—After separating from the 
main canal, the sub-orbital branch passes in a lateral direction 
into the pre-frontal and then through a series of circum-orbital 
bones, of which there are four, the second forming the 
posterior ventral angle, while the third and fourth are not 
visible externally. Leaving these bones as a dermal canal, it 
enters a series of drain-pipe-like bones by which it is conducted 
along the ventral and lateral borders of the rostrum. In the 
anterior portion it makes a slight lateral divergence and passes 
