170 MARINE SAURIANS. 
if composed of a single bone. Each side of the 
lower jaw was therefore made up of six separate 
pieces, set together in a manner that will be 
best understood by reference to the Figures in 
PL 11.* 
This contrivance in the lower jaw, to combine 
the greatest elasticity and strength with the 
smallest weight of materials, is similar to that 
adopted in binding together several parallel 
plates of elastic wood, or steel, to make a cross- 
bow ; and also in setting together thin plates of 
steel in the springs of carriages. As in the 
carriage spring, or compound bow, so also 
in the compound jaw of the Ichthyosaurus, 
the plates are most numerous and strong, at 
the parts where the greatest strength is required 
to be exerted ; and are thinner, and fewer, 
towards the extremities, where the service to 
be performed is less severe. Those who have 
* These figures are selected from various plates by Mr. Cony- 
beare and Mr. De la Beche. Fig. 1 is a restoration of the entire head 
of an Ichthyosaurus, in which each component bone is designated 
by the letters appropriated by Cuvier to the equivalent bones in 
the head of the Crocodile. In the lower jaw, u, marks the dental 
bone; v, the angular bone ; a;, superangular or coronoid; r/, arti- 
cular bone; z, complementary; ^, opercular. Fig. 2, is part 
of an under jaw of an Ichthyosaurus, showing the manner in 
which the flat bones, v, x, u, are applied to each other, towards 
the posterior part of the jaw. Figs. 3, 4, 5, 6, 7, show the 
manner in which these bones overlap, and lock into each other, 
at the transverse sections, indicated by the lines immediately 
above them in Fig. 2. Fig. 8, shows the composition of the 
bones in the lower jaw, as seen from beneath. 
