431 



Scylliura, Pristiurus, Mustelus, Galeus, and the two 

 Batoids, Rhynchobatus and S y r r h i n a. Van Bemmelen adds 

 Acanthias, Squatina and Heptanchiis to Müller's list. In 

 Acanthias, however, it is not the dorsal caecum, but the mucous 

 membrane of the spiracle itself, that is bound to the auditory cartilage. 

 The attachment, moreover, is eifected below and not above the post- 

 orbital groove. 



Fig. 2. Scyll ium cauicula. Transverse section of head, showing the relations 

 of the spiracular casca. 



VC position of ventral spiracular cjecum ; ch ceratohyal ; cbr first ceratobranchial ; 

 m mouth cavity; md mandible; pq palatoquadrate cartilage; sp external aperture of 

 spiracle; dc dorsal spiracular cacum ; pos post-orbital blood-sinus; hsc horizontal semi- 

 circular canal; cc cranial cavity; v vestibule of ear; hm hyomandibular cartilage. 



Projecting inwards and forwards from the inner wall of the spiracle, 

 near its pharyngeal opening, is another diverticulum, much shallower 

 in extent and with a wide mouth. It is situated below the level of 

 the pre-spiracular cartilage, and immediately in front of the proximal 

 end of the hyomandibular. It is bounded anteriorly by the levator 

 maxilkTe superior is muscle, and posteriorly by the anterior sur- 

 face of the hyomandibular cartilage and the inferior post-spiracular 

 ligament. The blind extremity of this ventral ca3cum is separated 

 only by the thickness of its mucous membrane from the orbital blood- 

 sinus. Van Bemmelen claims to have discovered a "Ventraler Spritz- 

 lochfollikel" which has exactly the relations of this ventral caecum, 

 except that it is united with the spiracular wall by a short stalk with 

 a narrow or no lumen. 



On comparing Mustelus, in which the ventral spiracular caecum 

 is very well shown, with Z y g ae n a and C a r c h a r i a s , the pharyngeal 



