MUSTELUS LJIVIS. 197 



referred to ; but it seems singular that the pseudol^ranch 

 should have acquired a position posterior to the adductor 

 hyomandibularis instead of lying- in front of it, where it 

 would naturally lie, and where it is found in Amia. 



The auditory diverticulum of Mustelus being simply the 

 dorsal end or pocket of the spiracular cleft, do the two 

 anterior or mesial diverticula of this fish represent similar 

 pockets of more anterior clefts ? 



Hoffmann (36, pp. 350 and 366) describes, in Acanthias, 

 what he calls the " dorsale Spritzlochanhang." This pocket 

 of the spiracular cleft is said to receive at its summit a 

 ventral branch of what Hoffmann designates as the " Ramus 

 accessorius " of the facial nerve, that ramus being said to be, 

 in all probability, the ramus oticus facialis of other authors. 

 This pocket in Acanthias must therefore be what I have 

 described as the dorso-mesial diverticulum of Mustelus. 

 Hoffmann considers this pocket in Acanthias as a remnant 

 of that branchial cleft that should be found between the 

 mandibular arch and Gregenbaur's second labial arch. He 

 then states that a still more anterior cleft, wdiicli must have 

 originally existed between the first and second labial arches, 

 has disappeared entirely, without even leaving a remnant. 

 If Hoffmann's interpretation of these pockets is correct, may 

 not the ventro-mesial diverticulum of Mustelus be such a 

 remnant ? 



Hoffmann and Wright both consider those tissues of the 

 spiracular diverticula of selachians that are innervated by the 

 so-called ramus accessorius of the one, and the ramus pretre- 

 maticus VII of the other, as of undoubted hypoblastic origin. 

 Both authors accordingly consider the nerve that is distributed 

 to these tissues as a ventral branch of a dorsal cranial nerve. 

 If the sensory organs here concerned are of hypoblastic 

 origin, they would seem necessarily to be homologous with 

 the sensory buds described by Alcock (1) on the diaphragm 

 of Ammocoetes. These organs of Ammocoetes are, however, 

 only found in the glossopharyngeal and vagus arches, as 

 I understand Alcock, and they are innervated by a nerve 



