MUSTELUS L*,VIS. 193 



The conclusion thus seems inevitable that it has its serial 

 homologue in the auditory diverticulum of Mustelus, that 

 diverticulum then being nothing more nor less than the dorsal 

 end, or dorsal pocket, of the spiracular cleft. The truncus 

 facialis in Mustelus lies always ventro-mesial to this diver- 

 ticulum, and the diverticulum nowhere presents a modified 

 epithelium such as is found in both the mesial diverticula. 



On the dorsal portion of the orbital wall of the skull, 

 ventro-mesial to the foramen by which the ramus oticus 

 facialis pierces the overhanging post-orbital process, and 

 directly external to the external semicircular canal of the 

 ear, a large and strong ligament has its origin. It is the 

 superior postspiracular ligament of Ridewood's descriptions, 

 and has already been several times referred to. Running 

 downward mesially and backward, external to the postorbital 

 blood sinus, it reaches the dorsal surface of the dorso-mesial 

 diverticulum of the spiracular cleft. There it spreads out, 

 and seems to separate into three somewhat separate parts. 

 One part continues ventro-mesially and is lost in a mass of 

 dense connective or fi.brous tissue that lies antero-mesial to 

 the dorso-anterior end of the diverticulum. A somewhat 

 separate part of this fibrous tissue directly surrounds the 

 distal end of the diverticulum, and into it a second part of 

 the ligament has its insertion. The third and larger portion 

 of the ligament continues downward, backward and laterally, 

 lateral to the spiracular pocket from which the two mesial 

 diverticula arise, and then posterior to the spiracular cleft 

 itself. At its distal end it separates into two parts, both of 

 which are inserted on the distal end of the hyomandibular, 

 near its anterior edge, or in the articular ligaments that bind 

 the hyomandibular and palato-quadrate cartilages together. 

 It passes, as already stated, between the truncus hyoideo- 

 mandibularis facialis and that trigeminal nerve that inner- 

 vates the levator maxillse superioris. The truncus facialis 

 lies between the ligament and the anterior edge of the car- 

 tilaginous hyomandibular, and this may give an explanation of 

 the varying and perplexing relations of the facial nerve to the 



