550 MR. T. G00DEY ON THE SKELETAL [Mar. 15, 



f. The Spiracle. 



The external opening of the left spiracle is an obliquely placed 

 slit on the postero-dorso-lateral surface of the head, measuring 

 about one centimetre in length. The right one is about one-half 

 this length. On removing the skin and carefully dissecting away 

 the underlying spongy cutis which covers the jaw-muscles, it is 

 seen that the lumen of the spiracle passes down into the oral cavity 

 between the hyomandibular and the mandibular cartilages. Just 

 inside the external opening, the cavity becomes enlarged and a 

 short caecal diverticulum is given off anteriorly. This is overlaid 

 by the levator maxillce, muscle whose disposition has already been 

 described. The caecum extends as far forward as the anterior 

 knob of the proximal end of the hyomandibular, which projects 

 from the articular depression on the auditory capsule. It is not 

 attached to the hyomandibular, but is separated from it by the 

 hyoidean branch of the seventh nerve, which passes just internal 

 and ventral to it. In all probability it is homologous with the 

 more extensive cseca mentioned by Ridewood (27), which have 

 been described in other Selachians by Midler and Van Bemmelen. 

 In Scyllium, for example, the caecum extends inwards over the 

 hyomandibular and becomes firmly attached to the wall of the 

 auditory capsule, being in some way concerned with the function 

 of hearing. A similar caecum is found in Heptanchus, so that 

 here we have another point in which Chlamydoselachus differs 

 from this member of the Notidanidae. The pseudobranch in each 

 spiracle consists of about ten short ridges, which lie on the 

 anterior outer wall just inside the external aperture. In the 

 Notidanidas the pseudobranchs are said to be better developed 

 than in any of the Selachians, so that in this respect we find 

 Chlamydoselachus presenting a small difference from Hexanchus 

 and Heptanchus. 



g. Features of Specialization and Comparison ivith Notidanidas. 



Perhaps the most important point in regard to the specialization 

 of the skull of Chlamydoselachus is to be seen in the extreme 

 length and mobility of the jaws. These are exceptionally long, 

 extending from the anterior, almost terminal mouth to a point 

 well behind the posterior limit of the cranium. This extension 

 is remarkable ; in fact, one quarter of the total length of the jaws 

 is found in this region, and it is this feature, connected with the 

 exceptional length of the hyomandibular, which gives the jaws 

 their great mobility. Indeed, their disposition relative to the 

 cranium is quite different from that found in any other Selachian 

 whose skull I have been able to examine or to see a figure of. It 

 resembles nothing among the Vertebrates so much, perhaps, as 

 the general disposition of the jaws in certain of the Ophidia. In 

 this respect also Chlamydoselachus presents a striking difference 

 from the two genera of the Notidanidae. In both Hexanchus and 

 Heptanchus the mouth is ventral and is situated far back. The 



