512 WALTER EDWARD COLLINGE. 
around the anterior border, joining with its fellow half of the 
opposite side in the median line (Pl. 39, fig. 11). 
The Hyomandibular Branch.—Leaving the main canal 
slightly posterior to the division resulting in the supra- and 
sub-orbital branches, the hyomandibular branch passes back- 
wards as a dermal branch over the region of the hyomandibular 
bone to the angle of the mouth, where it divides into two smaller 
branches of much less diameter, viz.a mandibular and a maxil- 
lary branch; the former is the larger, and passes along the 
mandible, the latter traversing the upper jaw for a short 
distance. Neither meet with the companion branches of the 
opposite side. Inthecourse of the hyomandibular branch from 
the dermo-sphenotic to the angle of the mouth there are twelve 
branches given off. They are similar in all appearances to 
those previously described on the lateral canal, only here there 
are dense patches of primitive pores also, in close connection 
with the terminations of the branches (Pl. 39, fig. 3). The 
branch @ shows both cluster and primitive pores at the termina- 
tions of the smaller branches. Judging from the two examples 
I have investigated of Polyodon, I should say that it is a 
feature of rare occurrence for a branch to terminate in primitive 
pores only, as in Pl. 39, fig. 3, d. 
3. THE CommissurES.—There are no true commissures 
present in the sensory canal system of Polyodon. In the 
numerous fishes in which this system has been investigated, the 
Polyodontidz are the only family in which as yet the com- 
missures are known to be absent. 
On either side of the occipital series of canal bones branches 
from the main canal pass inwards, as if to meet in the median 
line, but, leaving their bony channel, they divide into three 
series of cluster pores, those of the one side having no con- 
nection with their companions of the opposite side (Pl. 39, 
fig. 1). 
A similar condition is found in the region of the pre-orbital 
commissure. From the supra-orbital branch three smaller 
branches pass off as short bony channels, and are continued for 
a short distance as dermal canals, each terminating in a series 
