200 EDWARD PHELPS ALLIS, J UN. 



tion of the development of the diverticulum in Acanthias 

 seems, in fact, to indicate that this is what takes place. That 

 the internal ends of the clefts could have persisted, and that 

 the prebranchial facial nerves could have in any way slipped 

 forward over the aborted top of the cleft, seems excluded by 

 the fact that both a ligament and a branch of the oticus 

 facialis extend downward to the dorsal end of the cleft, 

 directly across the path that the nerves concerned must have 

 taken in so slipping forward. 



Truncus hyoideo-mandibularis facialis. 



The truncus hyoideo-mandibularis facialis, after its origin 

 from the dorso-posterior surface of the trigemino-facial gan- 

 glion, runs backward and upward along the side wall of the 

 skull, immediately anterior to the hind end of the peri-orbital 

 sinus. It is accompanied, in this part of its course, by the 

 r. ad. muse. lev. max. sup., as already fully described. Soon 

 after this trigeminal nerve turns outward, anterior to the 

 superior postspiracular ligament, the truncus facialis turns 

 outward posterior to that ligament, passing, as the trigeminal 

 nerve does, ventral to the postorbital blood-sinus. The 

 truncus facialis here runs dorsal to the posterior portion of 

 the dorso-mesial diverticulum of the spiracular cleft, and 

 antero-mesial to the dorsal end of the auditory diverticulum 

 of the same cleft. Its relations to the spiracular structures 

 are thus exactly those given by Ride wood (54) in his descrip- 

 tions of the corresponding parts of Scyllium. The truncus 

 then turns downward and laterally, passes immediately 

 posterior to the spiracular cleft, and reaches a position lateral 

 to the anterior edge of the distal end of the hyomandibular. 



Slightly distal to the point where the truncus thus turns 

 downward and laterally, a small ganglion forms on its hind 

 edge. From, or in connection with, this ganglion four 

 branches arise. Three of them run outward alono- the 

 anterior edge of the muscle Csdj, passing posterior to the 

 auditory diverticulum of the spiracular cleft, and postero- 



