Oct., 1891 . 
DRAGONS OF THE PRIME. 
221 
reptiles. The body of each vertebra (Fig. 6) is hollowed out at 
either end, in other words, it is biconcave. Now, such vertebrae 
are but rarely found in existing reptiles, but are very com¬ 
monly present in fishes and some amphibia—those huge 
forms, for instance, the Labyrinthodonts, which crawled 
about the marshy lands where our coal was formed. These 
biconcave vertebrae, then, show that Ichthyosaurus was con¬ 
nected with lower forms of animal life than reptiles. 
(2.) Again, in the skull (Fig. 4), the bones form a very 
complete roof, more so than in any existing reptiles, for the 
infra-temporal fossa is covered over by bones—post-orbital (Po.), 
supra-temporal (St.), and the quadrato-jugal (Qj.). This 
arrangement is not found in existing reptiles, but in extinct 
amphibia—the Labyrinthodonts just mentioned. 
(3.) The bones which make up the paddles (Fig. 3) are very 
simple in character, and are much less specialised than those of 
the limbs of most other reptiles (Fig. 2) and of amphibia. On 
the other hand, the bones of the fins in fishes which correspond 
to those of limbs of Ichthyosaurus are even less specialised than 
in the latter. It has been supposed, therefore, that the paddle of 
Ichtlivosaurus is a stage between the fin of the fish and the 
fully developed limb of the amphibian reptile. But there 
is some evidence to show that the paddle of Ichthyosaurus is 
not so primitive a feature as would at first sight appear, for it 
is said that the Triassic Ichthvosaurs had the humerus, ulna, 
and radius longer than in the Liassic species. If this be so, it 
shows that the Iclithyosaurian paddle is a secondary structure. 
While, then, Ichthyosaurus is undoubtedly a reptile, yet 
it shows connections with the lower groups of the fishes and 
amphibia. Nowadays we believe that the reptiles are the 
descendants of certain amphibia, and the amphibia to be the 
descendants of certain fishes. Hence, at first sight, we 
might imagine that in Ichthyosaurus we had the transitional 
form, the missing link between fishes and reptiles; and, since 
in structure he is partly fish, partly amphibian, partly reptile, 
we might think lie was the descendant of the former and 
ancestor of the latter. But this is probably not so. Ichthyo¬ 
saurus is, indeed, the descendant of a fisli-like ancestor, yet 
he is not the parent of other reptiles ; he is only nearer than 
other reptiles to that animal which is the common ancestor 
of them all. 
Now,let us examine into the structure of Plesiosaurus (Fig.7), 
a creature which derives its name from the fact that it is more 
like a lizard than Ichthyosaurus. In some respects it resem¬ 
bled the latter ; thus, it lived in the sea, had a smooth skin, 
and swam about by means of paddles. Hence, on account 
