446 The Evolution of Fishes 



Of the very earliest sharks in the Upper Silurian Age the 

 remains are too scanty to prove much save that there were sharks 

 in abundance and variety. Spines, teeth, fragments of shagreen, 

 show that in some regards these forms were highly specialized. 

 In the Carboniferous Age the sharks became highly varied and 

 extensively specialized. Of the Paleozoic types, however, all 

 but a single family seems to have died out, leaving Cestraciontes 

 only in the Permian and Triassic. From these the modem 

 sharks one and all may very likely have descended. 



Origin of the Sharks. — Perhaps the sharks are developed from 

 the still more primitive shark imagined as without limbs and 

 with the teeth slowly formed from modification of the ordinary 

 shagreen prickles. In determining the earliest among the 

 several primitive types of shark actualh^ known we are stopped 

 by 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 arclii pterygium, a series of consecutive 

 rounded cartilaginous elements with a fringe of rays along its 

 length. Sharks possessing this form of limb (IclitJiyoioini) 

 appear in the Carboniferous rocks, but are not known earlier. It 

 may be that from these the Dipnoans, on the one hand, may be 

 descended and, on the other, the true sharks and the Chimseras; 

 but there is no certainty that the jointed arm or archipterygium 

 of the Dipnoans is derived from the similar pectoral fin of the 

 Ichthyotomi. 



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 Cladoselache of the Upper Devonian, the 

 pectoral and the ventral fins are long and low, and arranged 

 just as they might be if Balfour's theory were true. Acan- 

 thoessus, 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 



