250 HISTORICAL PALEONTOLOGY. 



The Ichthyosauri or "Fish-Lizards" are exclusively Meso- 

 zoic in their distribution, ranging from the Lias to the Chalk, 

 but abounding especially in the former. They were huge 

 Reptiles, of a fish-like form, with a hardly conspicuous neck 

 (fig. 176), and probably possessing a simply smooth or 

 wrinkled skin, since no traces of scales or bony integumentary 

 plates have ever been discovered. The tail was long, and 

 was probably furnished at its extremity with a powerful ex- 

 pansion of the skin, constituting a tail-fin similar to that pos- 

 sessed by the Whales. The limbs are also like those of Whales 

 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 (vertebrae) 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 Ich- 

 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 



