484 AUGUSTA LA MONT ON 



it, and that there are only five canals in the dorsal inner hyoid group — a decidedly 

 lesser number than in other species. 



On the ventral surface (Plate V, fig. 8) the outer group of hyoid canals is shorter and 

 covers a less extended surface of the fin than it does in other members of the genus. 



Capsular walls were observed to be absent from the ophthalmic and inner and 

 outer buccal ampullae. 



R. clavata, Linn., or the " Thornback Kay," has the three or four canals in the 

 dorsal posterior hyoid group all of equal length and lying close together for the 

 whole of their course. The last two canals of the dorsal outer hyoid group terminate 

 at a greater distance from the border of the fin than do the canals next to them 

 (Plate VI, figs. 9 and 10). 



Specimens of this species also, dissected for the three cephalic groups of ampullae, 

 showed an absence of enclosing fibrous tissue. 



R. microcellata, Montagu. — The only specimen examined had in the group 

 selected for comparison three close and parallel canals of nearly equal length 

 (Plate VII, figs. 11 and 12). 



The last canal of the dorsal outer hyoid group ended at the angle where the 

 other canals cease to cross the muscle ridges and commence to follow the groove 

 between them. 



R. maculata, Montagu, has five canals in the dorsal posterior hyoid group, lying 

 close together for the whole of their course, and all of equal length (Plate VIII, 

 figs. 13 and 14). 



The features above enumerated have been selected for purposes of comparison 

 where distinguishing features are neither obvious nor easy to describe. It. is not, 

 however, suggested that these characters are necessarily constant, but rather that, 

 owing to the variability which the canals exhibit, especially as regards numbers, the 

 development of any given species might be expected to be in the direction indicated, 

 with fluctuations about an average type. 



5. Innervation. 



As stated above, all the five pairs of groups of ampullae are innervated by 

 branches of the facial nerve, and have been named after the branches by which they 

 are severally supplied. Each of these supplying branches innervates, in addition to 

 the ampullae, sense organs of the sensory canals, as described and figured by Ewart 

 in his paper on the Sensory Canals of the Common Skate, in which the relationship 

 of the two systems as regards their innervation is made clear. For facts relating to 

 the nerve-supply of the sensory canals reference has therefore been made to the 

 above paper. 



Since a dissection of the dorsal surface best exposes the nerve-supply of the 

 superficial ophthalmic, outer buccal, and hyoid ampullae, while the inner buccal and 



