276 , MB. W. E. COLLLNGE ON THE [Apr. 2, 



the frontal region ; it continued through the nasal and terminates 

 in the premaxilla. The sub-orbital branch passes through the 

 anterior border of the sphenotic and into the post-orbital, in which 

 it gives off a short branch and passes into the sub-orbital, giving off 

 another branch and also opening by a pore. It continues forwards 

 in the pre-orbital, in which in the suture v,ith the nasal there is 

 another pore ; it then passes through the nasal bone and terminates 

 in a bifurcation in the maxilla. 



Course of the Canals and Branches. 



1. The Lateral Canal is an exceedingly fine dermal canal, 

 scarcely visible to the naked eye in the posterior portion of the 

 body. In the anterior region its diameter increases slightly ; in 

 no portion, ho\\ever, was it found in transverse diameter to 

 exceed '6 of a millimetre. 



It commences at a small pore 5 mUlim. in front of the base of 

 the rays of the caudal fin. This terminal portion of the canal is 

 directed someM'hat dorsally, and therefore shares in the upward 

 flexure of the terminal part of the vertebral column, as in the tail 

 of Polyodon. It continues forwards along the side of the body, 

 rising upon the post-temporal region. Distributed along its whole 

 length are a series of pores, which are largest and most plentifid 

 in the anterior region. 



2. The Main Canal of the head commences in the squamosal. 

 At its commencement it gives off a lateral and backwardly directed 

 branch (PI. XVIIl. fig. 1) which ends bhndly, a feature common 

 to very many of the Physostomi. In front of this branch the 

 opercido-mandibular branch arises. The main canal then continues 

 forwards, passing through the dorsal border of the sphenotic. Its 

 com'se is not perfectly straight, as in many fishes, but has a slight 

 lateral inclination. In the most anterior portion of the sphenotic 

 the canal divides into supra- and sub-orbital branches. 



The Siqyra-orhiial Branch. — Leaving the main canal on the 

 anterior border of the sphenotic this branch passes forwards and 

 inwards in the frontal. At its commencement it gives off on its 

 inner side a backwardly directed branch, which passes into the 

 squamosal and terminates at pore 15 (fig. 1). In front of this 

 branch a much smaller one (pore 16) passes off and opens on the 

 dorsal surface of the head. Slightly in front of this, and about 

 the centre of the frontal bone, a branch (fig. l,f.com.) is given off 

 which meets \\ith its fellow of the opposite side, thus forming a 

 frontal commissure and connecting the supra-orbital branches of 

 either side. A similar commissure is present in Chcetostomus. 

 From this commissure the canal continues through the frontal 

 and lateral ethmoid, in the latter giving off a lateral and back- 

 wardly directed branch terminating at pore 17 (fig. 1), and passes 

 into the nasal, here also branching laterally. The main branch 

 passes forwards into the premaxillae, on the lateral border of which 

 it terminates at pore 19. 



