The Lateral Sensory System in the Muraenidae. 127 



a rounded eminence that lies abont niidway between the eye and the 

 anterior end of the snout. The posterior nasal aperture lies about 

 two-thirds the distance backward from the anterior aperture to the 

 anterior edge of the eye, and is a long- slit-like opening on the lateral 

 surface of the upper lip, close to its ventral edge. The dorsal edge 

 of this aperture projects ventrally as a strong flap and gives to the 

 aperture the appearance of opening on the ventral edge of the upper 

 lip. Such is however not the case, for it opens distinctly on the 

 lateral surface of the lip, close to its ventral edge. From there a 

 wide and flat posterior nasal passage runs upward and slightly forwai'd 

 to the nasal sac. 



The main infraorbital canal, beginning at the above-mentioned 

 pore, runs backward and inward and enters the anterior end of a 

 long tubular bone which runs backward internal to the posterior nasal 

 passage and then backward ventral to the anterior two-thirds of the 

 eye. The infraorbital canal traverses this bone its full length, giving 

 off in its passage but one primary tube, which perforates the bone 

 and opens on the outer surface by a single pore lying slightly poste- 

 rior to the posterior nasal aperture. Between this pore and the 

 anterior pore of the line two sense organs were distinctly evident in 

 the canal. One primary tube, the second of the line, must accordingly 

 have disappeared between these two organs, occluded by the descent 

 of the posterior nasal aperture from a position dorsal to the canal to 

 one ventral to it. That this must be what has taken place, and that 

 this descent of the aperture must have taken place after the lateral 

 canal was enclosed, is too evident to need discussion. The position 

 of the aperture, on the edge of the lip, cannot accordingly represent 

 a stage in an assumed rotation of the aperture from the mouth cavity 

 on to the external surface of the head, for the posterior nasal passage, 

 in that case, could not possibly acquire the relations it has to the 

 lateral canal and the bone that encloses it. 



At the hind end of this first bone of the infraorbital chain a 

 primary tube is given off, and opens on the outer surface by a single 

 pore. The canal then continues its straight and horizontal course, 

 traverses a second tubular bone, which is short and lodges but a 



