The Class Elasmobranchii or Shark-like Fishes 5 1 1 



Crossopterygians, the type called by Gegenbaur archipterygium 

 on the hyopthesis that it represents the primitive vertebrate 

 limb. 



In most sharks the fin has a fan-shape, with three of the 

 basal segments larger than the others. Of these the mesop- 

 terygium is the central one, with the propterygium before it 

 and the metapterygium behind. In the living sharks of the 

 family of Heterodontidce, this form of fin occurs and the teeth 

 of the same general type constitute the earliest remains dis- 

 tinctly referable to sharks in the Devonian rocks. 



Primitive Sharks. Admitting that these four types of pec- 

 toral fin should constitute separate orders, we have next to 

 consider which form is the most primitive and what is the 

 line of descent. In this matter we have, in the phrase of Haeckel, 

 only the "three ancestral documents, Paleontology, Morphol- 

 ogy, and Ontogeny." 



Unfortunately the evidence of these documents is incom- 

 plete and conflicting. So far as Palaeontology is concerned, 

 the fin of Cladoselache, with that of Acanthoessus, which may 

 be derived from it, appears earliest, but the modern type 

 of pectoral fin with the three basal segments is assumed to 

 have accompanied the teeth of Psammodonts and Cochliodonts, 

 while the fin of the Chimasra must have been developed 

 in the Devonian. The jointed fin of Cladodus and Pleura- 

 canthus may be a modification or degradation of the ordinary 

 type of shark-fin. 



Assuming, however, that the geological record is not perfect 

 and that the fin of Cladoselache is not clearly shown to be primi- 

 tive, we have next to consider the evidence drawn from mor- 

 phology. 



Those who with Balfour and others (see page 69) accept the 

 theory that the paired fins are derived from a vertebral fold, 

 will regard with Dean the fin of Cladoselache as coming nearest 

 the theoretical primitive condition. 



The pectoral fin in Acanthoessus Dean regards as a specialized 

 derivative from a fin like that of Cladoselache, the fin-rays 

 being gathered together at the front and joined together to 

 form the thick spine characteristic of Acanthoessus. This view 

 of the morphology of the fin of Acanthoessus is not accepted by 



