The Class Elasmobranchii or Shark-like Fishes 513 



arise from the same primitive stock, ancestors, or allies of the 

 Ichthyotomi, which group would also furnish the ancestors 

 of the Chimaras. In support of this view, the primitive pro- 

 tocercal or diphycercal tail of Pleuracanthus may be brought 

 in evidence as against the apparently more specialized hetero- 

 cercal tail of Cladoselache . But this is not conclusive, as the 

 diphycercal tail may arise separately in different groups through 

 degeneration, as Dollo and Boulenger have shown. 



The matter is one mainly of morphological interpretation, 

 and no final answer can be given. On page 68 a summary of 

 the various arguments may be found. Little light is given 

 by embryology. The evidence of Palaeontology, so far as it 

 goes, certainly favors the view of Balfour. Omitting detached 

 fin-spines and fragments of uncertain character, the earliest 

 identifiable remains of sharks belong to the lower Devonian. 

 These are allies of Acanthoessus. Cladoselache comes next in 

 the Upper Devonian. Pleuracanthus appears with the teeth 

 and spines supposed to belong to Cestraciont sharks, in the 

 Carboniferous Age. The primitive-looking Notidani do not 

 appear before the Triassic. For this reason the decision as 

 to which is the most primitive type of shark must therefore 

 rest unsettled for the present and perhaps for a long time 

 to come. 



The weight of authority at present seems to favor the view 

 of Balfour, Wiedersheim, Boulenger, and Dean, that the pec- 

 toral limb has arisen from a lateral fold of skin. But weight 

 of authority is not sufficient when evidence is confessedly 

 lacking. 



For our purpose, without taking sides in this controversy, we 

 may follow Dean in allowing Cladoselache to stand as the most 

 primitive of known sharks, thus arranging the Elasmobranchs 

 and rays, recent and fossil, in six orders of unequal value 

 Pleuropterygii, Acanthodei, Ichthyotomi, Notidani, Asterospondyli, 

 and Tectospondyli. Of these orders the first and second are 

 closely related, as are also the fourth and fifth, the sixth being 

 not far remote. The true sharks form the culmination of one 

 series, the rays of another, while from the Ichthyotomi the Cros- 

 sopterygians and their descendants may be descended. But 

 this again is very hypothetical, or perhaps impossible ; while, on 



