THE ELASMOBRANCH FISHES 



277 



and at tlie tip of the nose passes through to the ventral side. It then continues 

 backward to meet the infraorbital (ioc). This condition is unlike that in Hep- 

 tanchus (fig-. 228) in which the terminal part going to the infraorbital is 

 usuall}^ broken in its course. The infraorbital drops back of the eye to a ventral 

 position and then forward under the eye; after passing the ventral terminus 

 of the supraorbital, it turns inward toward the middle line, where it approxi- 

 mates or meets (Lae- 

 margus) the infraor- 

 bital of the opposite 

 side. It then continues 

 forward and slightly 

 outward to the tip of 

 the nose. The hyonian- 

 dibular canal {hmc.) 

 in the sharks branches 

 oi¥ posteriorly from 

 the infraorbital at the 

 place where the latter 

 reaches a ventral posi- 

 tion back of the eye. 

 Essentially the same 

 plan obtains in Squa- 

 lussucMii (fig. 241) as 

 that here described 

 for Laemargiis. 



This plan is consid- 

 erably modified in the 

 rays. In these, the su- 

 praorbital canal (soc, 

 fig. 242) in its ventral 

 course is characterized 

 by a peculiar loop for- 

 ward and outward. It 

 then meets the infra- 

 orbital ventrally as in 

 sharks. The hyoman- 

 dibular(/tmc..fig.242) 

 in the rays is greatly 

 modified. It passes from the infraorbital backward, outward, and then for- 

 w^ard on the ventral side of the pectoral fin, making a large ventral loop (in- 

 complete in Torpedo). It then perforates the fin at the side of the olfactory 

 capsule, and continues its course on the dorsal side of the fin, first inward and 

 backward; then it swerves far outward and backward and then inward to join 

 an anterior scapular l)ranch (sc.^) from the lateral canal, forming with the 

 scapular branch dorsally on the pectoral fin a characteristic loop. 



Fig. 243. Cephalic cauals in Dicerobatis, dorsal view. (From 

 Garman.) 



C.C., commissural or supratemporal canal ; hmc, hyoman- 

 dibular canal; ioc, infraorbital canal; II., lateral line; soc, 

 supraorbital canal. 



