178 MARINE SAURIANS. 
although united to a head nearly resembling 
that of a Lizard, assumed, in the leading prin- 
ciples of its construction, the character of the 
vertebrae of fishes. As this animal was con- 
structed for rapid motion through the sea, the 
mechanism of hollow vertebrae, which gives fa- 
cility of movement in water to fishes, was 
better calculated for its functions than the 
solid vertebrae of Lizards and Crocodiles.* 
(See Plate 12, a. and b.) This hollow coni- 
cal 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 
* The sections of the vertebrse of a iish (A c. c.) present two 
hollow cones, united at their apex in the centre of each verte- 
bra, in the form of an hour-glass ; but the base of each cone, 
(b. b.) instead of terminating in abroad 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 vertebrse, 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 mo- 
tion in every direction ; thus forming a kind of universal joint, 
and giving to the entire column great strength, and power of 
rapid flexion in the water. But as the inflections in the perpen- 
dicular direction are 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 ani- 
mals 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 which alone 
the vertebrse touch each other. 
