THE JURASSIC PERIOD. 243 



in the essentials of their structure, and in their being adapted 

 to act as swimming-paddles. Unhke 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 (vertebrse) also show the same adaptation to an aquatic 

 mode 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 Ic/i- 

 thyosaiu's 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 Ichthyosaiirtis which 

 have been discovered in the Lias indicate an animal of from 

 20 to nearly 40 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 Plesiosaurus (fig. 177) is another famous Oolitic 

 Reptile, and, like the preceding, must have lived mainly or 

 exclusively in the sea. It agrees wuth the Ichthyosaur in some 

 important features of its organisation, especially in the fact 

 that both pairs of limbs are converted into "flippers" or 



