ICHTHYOSAURUS. 177 
witnessed the shock given to the head of a 
Crocodile, by the act of snapping together its 
thin long jaws, must have seen how liable to 
fracture the lower jaw would be, were it com- 
posed of one bone only on each side : a similar 
inconvenience would have attended the same 
simplicity of structure in the jaw of the Ichthy- 
osaurus. In each case, therefore, the splicing 
and bracing together of six thin flat bones of 
unequal length, and of varying thickness, on both 
sides of the lower jaw, affords compensation for 
the weakness and risk of fracture, that would 
otherwise have attended the elongation of the 
snout. 
Mr. Conybeare points out a further beautiful 
contrivance in the lower jaw of the Ichthyosaurus, 
analogous to the cross bracings lately introduced 
in naval architecture, (see PL 11, Fig. 2.)'^ 
VertehrcE, 
The vertebral column in the Ichthyosaurus was 
composed of more than one hundred joints ; and 
* The coronoid bone, (x) is interposed between the dental, (u), 
and opercular (&), its fibres having a slanting direction, whilst 
those of the two latter bones are disposed horizontally; thus, 
the strength of the part is greatly increased by a regular dia- 
gonal bracing, without the least addition of weight or bulk ; a 
similar structure may be noticed in the overlapping bones of the 
heads of fish, and in a less degree, in those of Turtles. — Geol. 
Trans. Lond. Vol. V. p. 565, and Vol. I. N. S. p. 112. 
GEOL. N 
