ICHTHYOSAURUS. 139 



This contrivance in the lower jaw, to combine the great- 

 est elasticity and strength with the smallest weight of mate- 

 rials, 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 com- 

 pound bow, so also in the compound jaw of the Ichthyosau- 

 rus, 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 inconve- 

 nience 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 farther beautiful contrivance 

 in the lower jaw of the Ichthyosaurus, analogous to the 

 cross bracings lately introduced in naval architecture, (see 

 PI. 11, Fig. 2.*) 



the manner in which the fiat bones, v, x, u, are applied to each other, 

 towards the posterior part of the jaw. Figs. 3, 4, 5, 6, 7, show the man- 

 ner in which these bones overlap, and lock into each other, at the trans, 

 verse 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 tlie least addi- 



