THE EXTINCT SPINE-FINNED SHARKS 



2923 



The oldest and most primitive representatives of the entire subclass 

 Group are tne armored sharks of the Devonian and lower Carboniferous 

 epochs, especially characterised by the simple structure of their fins, 

 which, as explained on p. 2687, are of the fold type, and consist simply of a series 

 of parallel cartilaginous rods arising from a broad base. In many, but not all of 

 them, the granules 

 constituting the 

 shagreen of mod- 

 ern sharks coal- 

 esced so as to form 

 large shields pro- 

 tecting the body; 

 and these fish were 

 also armed with 

 more or less mark- 

 edly triangular 

 spines inserted in 

 the skin by their 

 bases. In some 



RESTORATION OF A FOLD-FINNED SHARK (Clddoselacfie). 

 (After Dean.) 



cases the teeth con- 

 sisted of a single 

 cone, with one 



small basal cusp; but in other forms they seem to have coalesced into a pavement- 

 like structure. No traces of calcification have been detected in the notochord. As 

 might have been expected these primitive sharks were of comparatively small size, 

 averaging from three to four feet in length. 



THE EXTINCT SPINE-FINNED SHARKS Order A CANTHODII 



Whereas the two preceding groups contain the most primitive and generalized 

 representatives of the subclass, the order now to be considered , which is likewise 

 confined to the Paleozoic epoch, comprises sharks of a more specialized type than 



any existing forms. 

 Indeed, these spine- 

 finned sharks bear 

 much the same relation- 

 ship to the lobe-finned 

 group, as is presented 

 by the bony fishes to 



RESTORATION OF SPINE-FINNED SHARK (Acanthodes). 

 (From A. S. Woodward, Cat. Foss. Fish. Brit. Mus.) 



the fringe-finned ga- 

 noids; and in a strictly 

 natural arrangement 

 these forms should stand at the head of the class, although it is more convenient 

 to consider them in this place. One of the essential features of the group is to be 



