SPINE-FINNED SHAPES. 
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 body; 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¬ 
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. 
restoration of a fold-finned shark (C'ladoselache). — After Dean. 
The Extinct Spine-Finned Sharks,— Order Acanthodii. 
Whereas 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 
any 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 
RESTORATION OF A SPINE-FINNED SHARK (AcCCnthodes). ... . 
(From A. S. Woodward, Cat. Foss. Fish. Brit. Mus.) the Ollgmal Caitllagill- 
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 
