PISCES . 121 



Pleuracanthus (Fig. 66) also contributes a very primitive fin charac- 

 ter but in another system, the caudal. Instead of having the hetero- 

 cercal type of tail fin which is characteristic of modern elasmobranchs, 

 it has a still more primitive one, a continuous fin-fold of the diphy- 

 cercal type, running smoothly about the tail. This is the type that 

 one finds in Amphioxus, in the hag-fishes, and, in a slightly modified 

 form, in the lampreys. It is also the earliest embryonic stage in the 

 median fin development of teleosts. There seems little question, then, 

 that the ancestral elasmobranchs had this kind of caudal fin. 



Putting together the most primitive characters of all of these ex- 

 tinct elasmobranchs we are able to describe a hypothetical ancestral 

 shark which is also probably the prototype of the earliest real verte- 

 brate. 



The Hypothetical Ancestor of the Elasmobranchs, and of 

 Fishes in General 



This creature must have had an elongated, spindle-shaped, fusi- 

 form body with terminal mouth, armed with dermal teeth, with prob- 

 ably more than seven gill-slits, with small lozenge-shaped dermal 

 denticles scattered over the skin, with lappet-like paired fins, and 

 a diphycercal tail-fin with low dorsal specializations of this fin-fold. 

 Internally, it probably had a persistent notochord with the merest 

 vestiges of vertebral arches. It also doubtless had lateral line organs 

 in open grooves, and, having no claspers, laid small eggs in the open 

 sea. The intestine probably had at least a primitive spiral valve. 



Some of the Specialized Modern Elosmobranchs of the Order 



Plagiostomi 



Sub-order 1. Selachii (true sharks) have remained on the whole com- 

 paratively unspecialized. For the most part they are active, free- 

 swimming, predaceous creatures such as the ancestral sharks must 

 have been. Among the more striking members of the Selachii are the 

 Hammer-Heads, the Whale-Sharks, and the Angel-Sharks. 



The Hammer-Heads (Sphyrnidce) are characterized by the lateral 

 protrusion of the eyes on large fiat stalks (Fig. 67, D) supported by 

 cartilaginous extensions of the cranium. There are all graduations 

 between the only slightly extended eyes to those in which the eyes ex- 

 tend so far as to give a width five times that of the nbrmal head. It 

 is not known that these greatly projecting eyes are of any; especial 



