THE EVOLUTION OF FISHES. 559 



an undetermined question of theory. What is the origin of paired 

 limbs ? Are these formed, like the unpaired fins, from the breaking up 

 of a continuous fold of skin, in accordance with the view of Balfour 

 and others? Or is the primitive limb, as supposed by Gegenbaur, a 

 modification of the bony gill-arch? Or again, as supposed by Kerr, is 

 it a modification of the hard axis of an external gill? 



If we adopt the views of Gegenbaur or Kerr, the earliest type of 

 limb is the jointed archipterygium, a series of consecutive rounded 

 cartilagenous elements with a fringe of rays along its length. Sharks 

 possessing this form of limb (Ichthyotomi) appear in the earliest 

 rocks, and from these the Dipnoi, on the one hand, may be descended 

 and, on the other, the true sharks and the Chimseras. 



On the other hand, if we regard the paired fins as parts of a lateral 

 fold of skin, we find primitive sharks to bear out our conclusions. In 

 Cladoselaclie of the Subcarboniferous, the pectoral and the ventral fins 

 are long and low, and arranged just as they might be if Balfour's theory 

 were true. Acantlioessus, with a spine in each paired fin and no other 

 rays, might be a specialization of this type or fin, and Climatius with 

 rows of spines in place of pectorals and ventrals might be held to bear 

 out the same idea. But in all these, the tail is less primitive than in 

 the Ichthyotomi. On the whole, however, there is much to be said on 

 the primitive nature of the Ichthyotomi, and Pleur acanthus, with the 

 tapering tail and jointed pectoral fin of a dipnoan, with other traits of 

 a shark, is as likely as Cladoselache to point directly to the origin of 

 the shark-like forms. 



Hasse finds this origin in a hypothetical group of Polyospondyli 

 which have many vertebra undifferentiated and without calcareous 

 material. These fishes are represented only by fin spines (Otichus), 

 which may have belonged to something else. These gave rise to 

 Ichthyotomi, with jointed fins, and through these to Dipnoi and a long 

 series leading to the bony fishes on the one hand and on the other to the 

 Amphibia, Eeptiles and Higher Vertebrates. 



The branch of higher sharks would lead to the Diplospondyli of 

 Hasse's system, of which Cladoselache should be a primitive example. 

 These sharks have the weakly ossified vertebrae joined together in pairs 

 and there are six or seven gill openings. This primitive type called 

 Notidani has persisted to our day, the frilled shark (Chlamydoselachus) 

 and the genera Hexanchus and Heptranchias, still showing its archaic 

 characters. 



Here the sharks diverge into two groups, the one with the vertebrae 

 better developed and its calcareous matter arranged star fashion. This 

 forms Hasse's group of Asterospondyli, the typical sharks. The earliest 

 forms (Heterodontidse, Hybodontidae) approach the Notidani, and one 

 such ancient type Heterodontus, still persists. The others diverge to 



