522 On the Fossil Fishes of the Carboniferous Limestone Series of Great Britain. 
tooth, the two are almost identical. The fossil examples do not appear to have 
had an osseous prolongation posteriorly for the muscular attachment of the upper 
and lower jaw, but this may be due either to the imperfection of the specimens, 
which are rarely perfect, or perhaps, more probably to difference in the organization 
of the fishes. Diodon is ranked amongst the Teleostean fishes, with an imperfectly 
ossified skeleton ; fins mostly soft, skin naked, except where it has developed 
osseous spines, the latter capable of extension for the fish’s protection, by the 
inflation of its body. The fossil genus Pristodus is represented only by its teeth, 
no other portion of its remains have been identified, and it may be inferred that it 
was a fish possessing a cartilaginous skeleton, and probably devoid of dermal spines 
or appendages. The skeleton being unossified, it naturally results that only the 
teeth are preserved in a fossil state. 
A comparison of the recent and fossil teeth, however, leads to a natural inference 
of relationship in some degree, however remote. Evidence is entirely wanting as 
to the anatomical structure of Pristodus, and I do not wish to lead to the inference 
that it was more nearly related than is warranted by the peculiar similarity of 
the teeth. There is a sufficiently special and peculiar adaptation of the jaws and 
teeth, of the two groups, to prove that it is not entirely an accidental one, we know 
that the teeth of the recent Diodon are admirably suited to the food on which it 
exists, for the purpose of breaking the hard coverings of corals or molluscs, and 
there is every reason to believe that those of the fossil forms were equally adapted 
to preying on the corals, crustacea and brachiopods existing in so great abundance 
in the carboniferous seas. 
Formation and locality : Yoredale Limestone, Richmond. 
Ez coll. Eavl of Enniskillen. 
Genus.—Cheirodus, M‘Coy. 
Cheirodus.—F. M‘Coy, 1848. “Ann. and Mag. Nat. Hist.,” 2nd ser., Vol. II., p. 131. 
‘General form of Ceratodus, that is, more or less fan-shaped, thick, flattened, 
with the anterior broad margin deeply divided into lobes; but the inner nearly- 
straight margin has a small recurved, thumb-like lobe projecting nearly at right 
angles from the middle of its length, preventing the mesial junction of the tritors 
of each side of the jaw: the inner marginal lobe is the longest ; surface minutely 
punctured.”—(I/‘Coy.) 
