ICHTHYOSAURUS. 135 



Head. 



The head, which in all animals forms the most important 

 and characteristic part, (see PL 10, Figs. 1,2,) at once shows 

 that the Ichthyosauri were Reptiles, partaking partly of the 

 characters of the modern Crocodiles, but more allied to 

 Lizards. They approach nearest to Crocodiles in the form 

 and arrangement of their teeth. The position of the nostril 

 is not, as in Crocodiles, near the point of the snout; it is set, 

 as in Lizards, near the anterior angle of the orbit of the eye. 

 The most extraordinary feature of the head, is the enormous 

 magnitude of the eye, very much exceeding that of any 

 living animal.* The expansion of the jaws must have been 

 prodigious; their length in the larger species, (Ichthyosaurus 

 Platyodon,) sometimes exceeding six feet ; the voracity of 

 the animal was doubtless in proportion to its powers of de- 

 struction. The neck was short, as in fishes. 



Teeth. 



The teeth of the Ichthyosaurus (PI. 11, b, c,) are conical, 

 and much like those of the Crocodiles, but considerably 

 more numerous, amounting in some cases to a hundred and 

 eighty ; they vary in each species ; they are not enclosed in 

 deep and separate sockets, as the teeth of Crocodiles, but are 

 ranged in one long continuous furrow, (PI. 11, b, c,) of the 

 maxillary bone, in which the rudiments of a separation into 

 distinct alveoli may be traced in slight ridges extending be- 

 tween the teeth, along the sides and bottom of the furrow. 

 The contrivance by which the new tooth replaces the old 

 one, is very nearly the same in the Ichthyosauri as in the 

 Crocodiles (PI. 11, a, b, c;) in both, the young tooth begins 

 its growth at the base of the old tooth, where, by pressure 



* In the collection of Mr. Johnson, at Bristol, is a skull of Ichthyosaurus 

 Platyodon, in which the longer diameter of the orbital cavity measures four- 

 teen inches. 



