18 SYDNEY EVANS JOHNSON 



the suprateniporal commissure and the dorsal series of pit 

 organs will be designated suprateniporal and dorsal rami of 

 the lateral nerve. 



All nerves of the sensory canal system of Mustelus and 

 Squalus occupy a position considerably removed from the canals 

 which they supply. These nerves give off numerous minute 

 branches which in turn make their way through the muscles 

 and connective tissue to the sensory epithelium of the canals. 

 These little branches will be termed the ramuli of the respective 

 nerves (fig. 4). 



IV. OBSERVATIONS 

 A. MUSTELUS CANIS 



1. Distribution of the canals and their gross innervation 



The supraorbital canal of the adult may be said to begin a 

 httle above the posterior angle of the eye where it is joined by 

 the infraorbital canal (fig. 2). It passes forward above the 

 orbit and a little in front of the latter it descends by a sharp 

 bend to a more ventral position. The canal then passes directly 

 to within a few millimeters of the tip of the snout, where it 

 curves .sharply ventralwards and backwards to merge with the 

 infraorbital canal posterior to the nostril (fig. 1). At its an- 

 terior extremity the supraorbital canal forms a union with the 

 anterior end of the infraorbital and, as indicated above, a second 

 anastomosis between these two canals is formed posterior to 

 the nostril (fig. 1). 



The supraorbital canal has more than a hundred tubules. 

 The tubules vary in length from 0.2 mm. to 1 mm. or more, 

 and the majority of them take a poster o-medial course to the 

 surface, where they open in simple pores. An exception to 

 this condition is found about half way between the orbit and 

 the snout. Here the tubules are double and for a short dis- 

 tance many of them branch and anastomose in a complex man- 

 ner. The significance of this condition is not apparent. It 

 naay indicate the fusion of two originally separate canals, or it 

 may represent a modification foreshadowing the complex sys- 



