a new Fossil Animal . 
573 
as Sir E. Home has well observed : he has also fully described the 
peculiarity of their spinal canal, the annular part not being consoli- 
dated with the body, as in quadrupeds, or connected by a suture as 
in Crocodiles, but remaining always distinct and united by a 
peculiar joint. The figures will sufficiently explain the form of 
the annular part and the joint in question 5 this structure is believed 
to be peculiar to the Ichthyosaurus.* 
The absence of transverse processes, and the consequent articu- 
lation of the ribs to the body of the vertebras by double lateral 
tubercles is another important distinctive feature. 
Their narrow form, or in other words, the small proportion 
which the length of the side of their body bears to the diameter of 
its articulating surface, is another good and ready criterion. In the 
Ichthyosaurus, the length of the side is less than half the diameter; 
in the vertebras of a true fossil Crocodile found at Gibraltar in 
Oxfordshire (the same apparently with the 2? fossil species of 
M. Cuvier,) it slightly exceeds the diameter; and in the Plesiosaurus, 
which holds an intermediate place in this respect, the ratio of the 
length of the side to the diameter is never so small as one half, and 
never so great as equality. Although the proportions may slightly 
vary in different parts of the column, the change is not sufficient 
to affect this general expression. 
The total number of vertebras has not been ascertained, but it 
* This mode of articulation of the annular part of the vertebrae with their bodies, 
by a regular joint, was necessary to cooperate with the cupped form of the intervertebral 
joints, in giving that flexibility to the vertebral column which the vibratory motions 
necessary in the mode of progression, which seems to have been common to this animal 
and fishes, required : for had these parts been consolitated as in quadrupeds, their 
articulating processes must have locked the whole column together so as to render such 
a motion of its parts impossible, but by means of this joint every part yields to that 
motion. 
