140 MARINE SAURIANS. 



VertehrcB. 



The vertebral column in the Ichtiiyosaurus was composed 

 of more than one hundred joints ; and although united to a. 

 head nearly resembling that of a Lizard, assumed, in the 

 leading principles of its construction, the character of the 

 vertebras of fishes. As this animal was constructed for ra- 

 pid motion through the sea, the mechanism of hollow verte- 

 brae, which gives facility of movement in water to fishes, 

 was better calculated for its functions than the solid verte- 

 brae of Lizards and Crocodiles.* (See Plate 12, a. and b.) 

 This hollow conical form would be inapplicable to the ver- 

 tebrae of land quadrupeds, whose back, being nearly at right 

 angles to the legs, requires a succession of broad and nearly 

 flat surfaces, which press with considerable weight against 

 each oth^r. It is quite certain, therefore, that such large 

 and bulky creatures as the Ichthyosauri, having their verte- 



tion of weight or bulk; a similar structure may be noticed in the over- 

 lapping bones of the heads of fish, and in a less degree, in those of Turtles. — ., 

 Geol. Trans. Lend. Vol. V. p. 565, and VoLI. N. S. p. 112. 



* The sections of the vertehrfB of a fish (A c. c.) present two hollow 

 cones, united at their apex in the centre of each vertebra, in the form 

 of an hour-glass; but- the base of each cone, (b. b.) instead of termi- 

 nating in a broad flat surface, like the base of the hour-glass, is bounded by a 

 thin edge, like the edge of a wine-glass, and by this alone touches the 

 corresponding edge of the adjacent vertebra. Between these hollow 

 vertebrae, a soft and flexible intervertebral substance, in the form of a 

 double solid cone (e. e.) is so placed that each hollow cone of bone plays 

 on the cone of elastic substance contained within it, with a motion in 

 every direction; thus forming a kind of universal joint, and, giving to the 

 entire cohimn great strength, and power of rapid flexion in the water. 

 But as the inflections in the perpendicular direction arc less necessary 

 than in the lateral, they are limited by the overlapping, or contiguity of the 

 spines. 



This mode of articulation gives mechanical advantage to animals like 

 fishes, whose chief organ of progressive motion is the tail ; and the weight of 

 whose bodies being always suspended in water, creates little or no pressure, 

 on the edges, by wliich alone the vertcbjx touch each other. 



