166 



PICTURESQUE SKETCHES. 



able. It may be necessary to remind the reader, 

 that in quadrupeds generally, and in birds, these 

 vertebrae not only present flat surfaces to one another, 

 but are more or less firmly con- 

 nected by the interlocking of pro- 

 jecting bones or processes. In the 

 reptiles generally there is a recess 

 on one side of each surface, consist- 

 ing of a cup-shaped Jiollow into 

 which a corresponding ball of the 

 adjacent vertebra fits and works. 

 In the Plesiosaurus, as I have al- 

 ready described, this is absent, and 

 the bones present simple, flat, or 

 very slightly doubly concave sur- 

 faces to each other; but in the 

 Ichthyosaurus and in fishes the 

 two cup-shaped hollows are deep 

 and strongly marked (figs. 60, 61), so that there was 

 considerable interspace between the centre of two 

 Fig. 61 adjacent vertebrae. This interspace is 

 filled with fluid in the case of fishes 

 and the Ichthyosaur, allowing of those 

 minute but incessant and very impor- 

 tant lateral motions by which the fish, 

 supported and floating in the water 

 without effort, moves itself readily in 

 every direction. In this respect, therefore, the 

 Ichthyosaur had the advantage of all reptiles, for, 

 being an inhabitant of the sea, it was provided with 

 the same power of ready motion and great flexibility 

 as the fishes themselves. 



But still this strange monster was a reptile. To 



ICHTHYOSAURUS. 

 (Vertebra.) 



