PISCES. 
61 
Carboniferous Limestone inclusive, are sometimes compared Table-ease 
with the teeth of lampreys and hag-fishes, but their exact E - 
nature is very doubtful. Specimens are exhibited from the 
Lower Carboniferous of Ohio, U.S.A. (Table-case E). 
Class VI.—PISCES. 
The earliest true fishes, with a well-formed lower jaw and 
paired fins, are represented by rare and fragmentary remains 
at the top of the Silurian rocks. Better-preserved specimens 
in the Lower Old Bed Sandstone show them to have 
possessed only a cartilaginous internal skeleton, and no bone- 
cells in their external armour. They therefore probably 
belong almost to the same grade as the existing sharks 
(Elasmobranchii). The first fishes with a gill-cover and with 
bony tissue in their skeleton occur in the Middle Old Bed 
Sandstone or Middle Devonian; and between these and 
Fig. 57. — Protocercal or Fig. 58. — Heterocercal Fig. 59. — Homocercal 
diphycercal tail; primi- (unequal-lobed) tail: mid- (equal-lobed)tail; modern 
tive type.| die type. type. 
modern fishes with a bony internal skeleton there are all 
gradations in successive geological formations. In the 
course of evolution it is interesting to observe that the tail 
undergoes considerable change. In all the older fishes the 
hinder extremity of the body tapers, and is either straight 
(Eig. 57) or with the fin almost or completely confined to 
the lower border (Eig. 58). In later fishes, the upturned 
end of the body in the unequal-lobed tail is more and more 
shortened, and the fin-rays gradually become so disposed that 
to all external appearance the tail assumes perfect symmetry 
(Eig. 59). Such changes are precisely repeated in the 
embryonic history of each existing bony fish; so that in the 
tail the history of the whole race corresponds with the 
history of each of its latest and highest individuals. 
