236 ON THE FOSSIL ICHTHYOSAURUS. 



contain a large proportion of lime, in the form of phosphate and car- 

 bonate. The rarity of coprolites at Barrow, and their abundance 

 at the mouth of the Severn, may be referable to certain differences 

 in the nature of the food of the Ichthyosauri in these localities, 

 that which contained the greatest proportion of bone and calcareous 

 matter, affording, of course, the largest quantity of coprolite. 



The structure of the head, teeth, and jaws, closely resembles that 

 of the true Crocodiles, as distinguished from the Alligators. These 

 are piscivorous, and, like the Porpoise, have their lower jaw in a 

 single piece ; but the Crocodiles, being bolder animals, and attack- 

 ing their prey either on land or in the water, require a stronger 

 construction of the two lower jaws, which, in order to fulfil this de- 

 sign, are composed of six bones on each side, so spliced together as 

 to give the greatest degree of lightness and firmness combined. The 

 number of teeth, which is about one hundred and eighty, is greater 

 than in the Ci'ocodile ; and they are not enclosed in sockets, but lie 

 in a groove of the jaw. As this structure renders them insecure, 

 and liable to be displaced, in order to replace those that are broken 

 or torn out, a constant succession of teeth is provided, that lie in a 

 depression in the side of each of the old ones, which they push out 

 by their growth, unbroken ; whereas the new tooth of the Croco- 

 dile, beiny contained within the old one, splits it before it expels it 

 from the jaw. The teeth and jaws form, at present, the specific 

 distinctions of the Ichthyosauri ; thus, we have the Platyodon and 

 the Tenuirostris, &c. ; but ]\Ir. Hawkins and Mr. Owen consider 

 that the paddles jiresent more unfailing characters for the establish- 

 ment of species. The nostrils, being placed almost immediately in 

 front of the eyes, indicate that, like the Pike, the Ichthyosaurus 

 fished by sight rather than by scent, and was probably equally vora- 

 cious as that fish. Indeed, if we may reason from the size of the e3'^e, 

 which is very large in most predacious animals, it must have been the 

 tyrant of the deep ; for these organs are larger, in proportion to the 

 head, than in any other of the finny tribes, except in a few of the 

 smaller fishes. The sclerotic coat of the eye was composed, as in 

 birds and crocodiles, of bony plates, that, ranged in a circle like the 

 staves of a barrel, and being movable over each other, by their con- 

 traction and expansion increased or diminished the axis of the eye, and 

 altered its range of vision from near to distant objects in air or water, 

 according as the huge monster floated on the surface or remained sub 

 merged; when, by the immense quantity of light admitted through 

 the enormous pupil, the deepest recesses of the ocean, or its most 

 turbid water, were subject to its ken. Although the structure of 



