THE SENSORY CANAL SYSTEM OF FISHES. 511 
branches were always in the form of single papille. In the 
dead Polyodon which had been in alcohol for some years, 
the apex of the papilla was hard and glossy as if for pro- 
tection. 
2. THE Main Canat or THE HEAv.—The main canal is a 
direct dermal continuation of the lateral canal. On the lateral 
border of the post-temporal region it passes on to the head as 
a dermal canal, being continued in a canal bone. It passes 
forwards along the dermo-sphenotic, on which bone it gives off 
a short lateral branch which terminates in a cluster pore. 
Wing-like processes extend over the anterior portion of the 
dermo-sphenotic, by which the canal is divided into three 
branches—the supra- and sub-orbital, both dorsal, and a pos- 
terior ventral one, the hyomandibular. 
The Supra-orbital Branch.—Leaving the main canal on 
the inner border of the anterior portion of the dermo-sphenotic, 
it passes forwards over the posterior lateral border of the 
frontal, giving off in its course three small branches (PI. 39, 
fig. 1) which continue as dermal canals for a short distance, 
and then each divides into still smaller branches, all of which 
open to the surface by a series of cluster pores. From the 
frontal bone the branch then takes a downward course, across 
the dermo-ect-ethmoid suture, between the nasal openings, and 
joins the sub-orbital branch. 
The Sub-orbital Branch.—The sub-orbital branch passes 
off from the main canal on the outer border of the anterior por- 
tion of the dermo-sphenotic, and is conducted around the pos- 
terior and inferior borders of the orbit in a series of small canal 
bones. On the ventral surface beneath the dermo-ect-ethmoid 
it joins with the supra-orbital branch, and the two are con- 
‘ducted as one canal in a series of canal bones which pass 
forwards and inwards towards the parasphenoid and continue 
along its ventral and lateral border. In their course forwards 
two lateral branches pass off, and break up in a series of very 
minute dendriform branches. 
Approaching the anterior portion of the rostrum the dia- 
meter of the canal becomes less, diverges laterally, and passes 
