SPINE-FINNED SHARKS. 



547 



which, as explained on p. 319, 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 con- 



stituting the sha- 

 green of modern 

 sharks coalesced so 

 as to form large 

 shields protecting 

 the bod} T ; and 

 these fish were 

 also armed with 

 more or less mark- 

 edly triangular 

 spines inserted in 

 the skin by their 

 bases. In some 

 cases the teeth con- 



restoration of a fold-finned shark (Cladoselache). — After Dean. 



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 3 to 4 feet in length. 



The Extinct Spine-Finned Sharks, — Order Acanthodii. 



AYhereas the two preceding groups contain the most primitive and generalised 

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

 confined to the Palaeozoic epoch, comprises sharks of a more specialised type than 

 i\ny existing forms. Indeed, these spine-finned sharks bear much the same 

 relationship to the lobe-finned group, as is presented by the bony fishes to the 

 fringe-finned ganoids ; and in a strictly natural arrangement these forms should 



stand at the head of the 

 class, although it is 

 more convenient to con- 

 sider them in this place. 

 One of the essential 

 features of the group 

 is to be found in the 

 development of mem- 

 brane-bones overlying 

 the original cartilagin- 

 ous skull; the socket 

 of the eye being also frequently surrounded with a ring of bones of similar 

 origin. In the internal skeleton the notochord is persistent, and the cartilages are 

 superficially calcified, frequently with a granular structure. When teeth are 

 present, these are firmly fixed upon membrane-bones overlying the cartilages corre- 

 sponding to the functional jaws of other sharks. The gill-arches bear a series of 



RESTORATION OF A SPINE-FINNED SHARK (Acfrathodcs). 



(From A. S. Woodward, Cat. Foss. Fish. Brit. M 



