CLASSIF1CA TION 



123 



The number of branchial slits is larger in some Chondrichthyes 

 (Notidani) than in any other Pisces, and this may be a primitive 

 character. We find, also, that 

 among the least differentiated and 

 earliest representatives the upper 

 jaw (palato-quadrate) is of ten broad 

 behind, and articulated by an otic 

 process to the auditory capsule. 

 It is possible that this amphistylic 

 type of jaw-articulation was pos- 

 sessed by the common ancestor 

 of the whole group of Chondrich- 

 thyes (p. 95). In the Elasmo- 

 branchii, and apparently also in 

 the Pleuracanthodii, the gill-septa 

 are supported by a single posterior 

 series of cartilaginous gill-rays 

 (Figs. 57, 99). If, as seems prob- 

 able, a similar single series of rays 

 existed in the Cladoselachii, this 

 character might distinguish the 

 whole Division /rom the Osteich- 

 thyes, in which there are two 

 series. But there is some reason 

 to believe that this was not the case 

 in the Acanthodii (p. 190). 



The Chondrichthyes retain 

 most of the structures mentioned 

 above as being characteristic of 

 primitive fish: the absence of true 

 bone, of scales other than den- 

 ticles, and the formation of the jaws from the palato-quadrate bar 

 and Meckel's cartilage, distinguish them from the rest of the Pisces. 



The classification of the Chondrichthyes still presents many diffi- 

 culties. The first two sub-classes described below (Elasmobranchii 

 and Pleuracanthodii) have many characters in common, and are 

 distinguished from all other Pisces by the development in the male 

 of copulatory 'claspers.' These are specialised posterior portions 

 of the pelvic fins. It seems very unlikely that the claspers should 

 have been developed independently in the Elasmobranchs and in 

 the Pleuracanthodians. 1 At the same time, to unite these two 

 groups into one sub-class would perhaps be to separate them too far 

 from the Cladoselachii. Until the structure and affinities of the 

 latter are better understood, it cannot be decided whether they are 



1 For a discussion of the morphology of the skeleton of the paired fins, see 

 pp. 73, 106 ; for the copulatory claspers, see p. 129. 



FIG. 83. 



Diagram of a section through the dorsal 

 fin of ticyllium. ct, ceratotrich ; d, denticle ; 

 radial. (From Quart. Jovrn. 



