432 



pit of the latter, usually regarded as the inner remnant of the closed 

 spiracle, would appear to be the ventral cascum rather than the inner 

 part of the true spiracular cavity. The relation of this pit to the 

 hyomandibular, the inferior post- spiracular ligament, and the orbit, 

 are exactly those of the ventral cöccum of Must el us. Müller i'^) 

 records the presence of a pseudobranch in the pharyngeal pit of 

 Carcharias, but this I am unable to confirm. 



In several species of Selachians there exists a pit or depression 

 of well-defined outline, situated in the floor of the mouth, between the 

 ceratohyal and the hind end of the mandible. The pit is shallow 

 and wide-mouthed, and the mucous membrane lining it is destitute 

 of the minute denticles which, in some genera, e. g. Gale us, are 

 embedded in the greater part of the mucous membrane of the mouth. 

 This pit is present in Galeus, Carcharias and Zygiena. It 

 is well developed in Triads, but only faintly marked in the closely 

 allied Mustelus: it is large in Chiloscyllium but absent in 

 Scy Ilium. It is also absent in Notidanus and Acanthias. On 

 taking a general survey of the pharyngeal openings of the gill clefts, 

 one cannot fail to notice that this pit is situated immediately in front 

 of the lower part of the first branchial cleft. If a line be drawn 

 joining the lower ends of the pharyngeal apertures of the branchial 

 clefts, it will pass through the lower or anterior extremity of this 

 pit, just as a curved line joining the upper ends of the branchial 

 clefts will, if produced, pass through the inner or superior edge of the 

 pharyngeal aperture of the spiracle. It is universally admitted that 

 the spiracle of sharks represents only the upper part of the hyoid 

 cleft, the middle and lower portions being obliterated. Here, in this 

 depression of the mucous membrane, is a structure, which, in complete 

 absence of evidence to the contrary, may be regarded as the internal 

 or pharyngeal portion of the lower half of the hyoid cleft. 



The existence and signification of this pit have not escaped notice 

 in the past, for, in 1886, Dohen described and figured*^), in an 

 early Torpedo embryo, a "Pseudobranchialrinne" or ventral, ento- 

 dermal part of the spiracular cleft. He was, however, of opinion that 

 the "Pseudobranchialrinne" was present only in the embryo, for he 

 definitely states in his preface (p. 5), "daß die vor ihm liegeilde Kiemen- 

 spalte, das Spritzloch, in ihrer Ausdehnung beschränkt ist, und nur 



12) 1. c. p. 237. 



13) Mitt. Zool. Stat. Neapel, Bd. VI, Taf. IV, Fig. 7. 



