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 crossbow; 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 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 composed of one bone only on each side: a similar inconvenience would have attended the same simplicity of structure in the jaw of the Ichthyosaurus. 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.)[1]
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.
- ↑ The coronoid bone, (x) is interposed between the dental, (u,) and opercular (&,) its fibres have 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 diagonal bracing, without the least addi-
