360 
ELEMENTS OF GEOLOGY. 
resembling that of a movable mast, raising and lowering 
backward the sail of a barge.”* 
Reptiles of the Lias. —It is not, however, the fossil fish 
which form the most striking feature in the organic remains 
of the Lias; but the Egialiosaurian reptiles, which are ex¬ 
traordinary for their number, size, and structure. Among 
the most singular of these are several species of Ichthyo¬ 
saurus and Plesiosaurus (Figs. 380, 381). The genus Ich¬ 
thyosaurus^ or fish-lizard, is not confined to this formation, 
but has been found in strata as high as the White Chalk of 
England, and as low as the Trias of Germany, a formation 
which immediately succeeds the Lias in the descending order. 
It is evident from their fish-like vertebrae, their paddles, re¬ 
sembling those of a porpoise or whale, the length of their 
tail, and other parts of their structure, that the. Ichthyosaurs 
were aquatic. Their jaws and teeth show that they were 
carnivorous; and the half-digested remains of fishes and 
reptiles, found within their skeletons, indicate the precise 
nature of their food. 
Mr. Conybeare was enabled, in 1824, after examining many 
skeletons nearly perfect, to give an ideal restoration of the 
osteology of this genus, and of that of the Plesiosaurus,^ 
(See Figs. 380, 381.) The latter animal had an extremely 
long neck and small head, with teeth like those of the croco¬ 
dile, and paddles analogous to those of the Ichthyosaurus 
but larger. It is supposed to have lived in shallow seas and 
estuaries, and to have breathed air like the Ichthyosaur and 
our modern cetacea.J Some of the reptiles above mentioned 
were of formidable dimensions. One specimen of Ichthyo¬ 
saurus platyodon^ from the Lias at Lyme, now in the British 
Museum, must have belonged to an animal more than 24 feet 
in length; and there are species of Plesiosaurus wdiich meas¬ 
ure from 18 to 20 feet in length. The form of the Ichthyo¬ 
saurus may have fitted it to cut through the waves like the 
porpoise; as it was furnished besides its paddles with a tail- 
fin so constructed as to be a powerful organ of motion; but 
it is supposed that the Plesiosaurus,^ at least the long-necked 
species (Fig. 381), was better suited to fish in shallow creeks 
and bays defended from heavy breakers. 
It is now very generally agreed that these extinct saurians 
must have inhabited the sea; and it was urged that as there 
are now chelonians, like the tortoise, living in fresh water, 
* Bridgewater Treatise, p. 290. 
t Geol. Soc. Transactions, Second Series, toI. i., p. 49. 
i Conybeare and De la Beche, Geol. Trans., First Series, vol. v., p. 559; 
and Buckland, Bridgewater Treatise, p. 203. 
