THE JURASSIC: PERIOD. 243 
in the essentials of their structure, and in their being adapted 
to act as swimming-paddles. Unlike the Whales, however, 
the Ichthyosaurs possessed the hind-limbs as well as the fore- 
limbs, both pairs having the bones flattened out and the fin- 
gers completely enclosed in the skin, the arm and leg being at 
the same time greatly shortened. The limbs are thus con- 
verted into efficient ‘‘ flippers,” adapting the animal for an 
active existence in the sea. ‘The different joints of the back- 
bone (vertebra) also show the same adaptation to an aquatic 
nfode of life, being hollowed out at both ends, like the bicon- 
cave vertebrae of Fishes. The spinal column in this way was 
endowed with the flexibility necessary for an animal intended 
to pass the greater part of its time in water. ‘Though the /cA/- 
thyosaurs are undoubtedly marine animals, there is, however, 
reason to believe that they occasionally came on shore, as they 
possess a strong bony arch, supporting the fore-limbs, such as 
would permit of partial, if laborious, terrestrial progression. 
The head is of enormous size, with greatly prolonged jaws, 
holding numerous powerful conical teeth lodged in a common 
groove. The nature of the dental apparatus is such as to 
leave no doubt as to the rapacious and predatory habits of the 
Ichthyosaurs—an inference which is further borne out by the 
examination of their petrified droppings, which are known to 
geologists as “coprolites,” and which contain numerous frag- 
ments of the bones and scales of the Ganoid fishes which 
inhabited the same seas. ‘The orbits are of huge size; and as 
the eyeball was protected, like that of birds, by a ring of bony 
plates in its outer coat, we even know that the pupils of the 
eyes were of correspondingly large dimensions. As these bony 
plates have the function of protecting the eye from injury 
under sudden changes of pressure in the surrounding medium, 
it has been inferred, with great probability, that the Ichthy- 
osaurs were in the habit of diving to considerable depths in 
the sea. Some of the larger specimens of /chthyosaurus which 
have been discovered in the Lias indicate an animal of from 
20 to nearly 4o feet in length ; and many species are known to 
have existed, whilst fragmentary remains of their skeletons are 
very abundant in some localities. We may therefore safely 
conclude that these colossal Reptiles were amongst the most 
formidable of the many tyrants of the Jurassic seas. 
The Plestosaurus (fig. 177) is another famous Oolitic 
Reptile, and, like the preceding, must have lived mainly or 
exclusively in the sea. It agrees with the Ichthyosaur in some 
important features of its organisation, especially in the fact 
that both pairs of limbs are converted into ‘ flippers” or 
