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GALLERY No. 6.—FOSSIL FISHES. 
As fishes are aquatic animals and as most fossiliferous 
rocks have been formed in water, fish-remains are naturally 
very abundant among fossils. The geological record of their 
past history, however, is much more imperfect than might 
have been expected; for almost the only good specimens 
are those obtained from shoals which have been suddenly 
destroyed and quickly buried. Our real knowledge therefore 
depends on a succession of local accidents, which reveal only 
isolated episodes instead of a continuous story. Even these 
episodes are incompletely recorded, because the skeletons 
of a large proportion of the lower fishes are too little hardened 
with lime (or “ calcified ”) to become fossilised when buried 
in rock. 
Class V.—AGNATHA. 
It is probable that all the earliest fish-like animals were Table-oases 
destitute of hard parts capable of fossilisation, because no A-Ei 
links have yet been found between fishes and the inverte¬ 
brate animals below. When they first appear in Upper 
Silurian rocks their fossilised remains merely represent skin- 
armour, so that it is difficult to ascertain precisely the nature 
of their organisation. There is not much doubt, however, 
that the forerunners of the fishes lacked both a lower jaw as 
ordinarily fashioned and paired fins corresponding with 
the arms and legs of land animals. They are therefore 
arranged in a distinct Class of Agnatha (“ without jaws ”) 
below that of Pisces (or fishes proper). These primitive 
animals occupy some of the small Cases in the middle of 
Gallery 6. 
Order I.— OSTRACODERMI. 
Nearly all known Agnatha of the Silurian and Devonian 
periods are armoured with hard skin-tubercles, which are like 
the placoid scales of sharks, but often united into plates by 
