64 |i GUIDE TO THE FOSSIL REPTILES, AMPHIBIANS, FISHES. 
Wall-case (Climatius, Parexus, etc.) have comparatively broad fin- 
1 - spines obviously formed by the fusion of rows of hard tuber- 
Table-case . an( j t p ere are p a i rs 0 f these spines on the lower border 
of the fish between the pectoral and pelvic fins, as if the 
paired fins had originally been a pair of continuous 
membranes, afterwards sub-divided. The Middle and 
Fig. 61.—Fin-spine of Gyracanthus formosus , from the Coal Measures of 
Dalkeith; about one-third nat. size. (Table-case 1.) 
Upper Devonian Piylacanthus and the Carboniferous and 
Permian Acanthodes (Fig. 60) are characterised by more 
slender fin-spines with little or no trace of intermediate 
paired spines. Gyracanthus is a curious Carboniferous genus, 
comprising comparatively large species, which are scarcely 
known except by their fin-spines (Fig. 61). 
Order II.— PLEUROPTERYGII. 
Wall-case Wall-case 1 contains some fine specimens of an Upper 
Table’case Devonian shark, Cladoselache , representing another primi- 
2 tive group, in which the teeth are loosely arranged in the 
jaws as in modern sharks, while the paired fins are mere 
balancers supported by separate parallel rods of cartilage. 
As in Acanthodians, there is a ring of plates round the eye. 
The tail is heterocercal, though it is less conspicuous in the 
fossils than the horizontal keel of skin which extends along 
each side of its base. The nearly complete examples of 
Cladoselache have been discovered only in the Cleveland 
Shale of Ohio, U.S.A., the largest being 5 or 6 feet in 
length; but the teeth in this fish closely resemble those 
named Cladodus , which are commonly found isolated in 
the Lower Carboniferous both of America and Europe 
(Table-case 2), and probably belong to allied genera. 
In some of the Palaeozoic sharks the piercing or cutting 
teeth succeeded each other rapidly during life as in existing 
sharks, but did not fall from the outer edge of the mouth 
when they were no longer wanted. The used teeth of each 
transverse row united into an ever-increasing coil outside the 
