structure admirably adapted to the mode of life of the animal—the climbing of trees and shrubs in pursuit of insects. The ichthyosaurus and plesiosaurus, creatures of the lizard tribe which have now become extinct, are both characterised by the form of their extremities. The former has four members like paddles, each composed of nearly a hundred small bones of an irregular polygonal shape and arranged in a pavement-like form. The latter has also four paddle-shaped extremities, but the number of bones of which each is composed is much less than in the ichthyosaurus—not above fifty, the pieces being at the same time of a different shape, and not arranged in a tessellated form. _Birds._—Among birds the foot is a most important organ, serving at once for support, prehension, and locomotion. Its modifications are accordingly very numerous, and ornithologists have employed the characters presented by it as a basis of classification. Scopoli, in his Introduction to Natural History (1777), proposed a systematic distribution of birds into two divisions founded upon the form of the scales covering the foot; thus, some are furnished with scales, small and polygonal (_Retepedes_), while others (_Scutipedes_) have the legs covered in front with unequal semicircular plates. So, again, in the division of birds proposed by Nitzsch, viz. into the aerial, terrestrial, and aquatic, the foot will at once serve to indicate the different orders. We will proceed to examine the special characters afforded by the foot of birds in each of the groups or orders, adopting the classification of Kirby, one of the most recent. In the first order, the birds of prey (_Raptores_), which includes the eagle, the vulture, the owl, &c., for the purpose of striking their quarry, and of securing it by a firm grasp, a strong muscular foot is necessary, and we consequently find that the foot is provided with four short muscular toes, three placed before, and one behind, all armed with long, sharp, hooked talons, those of the thumb and innermost toe being the longest, as the great duty of grasping is mainly performed by them. The claws are also capable of being drawn back, they are retractile like those of the cat; and the scales upon the upper surface of the foot, as well as the skin covering it below, are strong and tough for the purpose of protecting the member from the attacks of the living prey upon which these creatures feed. The foot is also broadly marked, the kind of marking and the size of the scales being especially characteristic of the different genera and families. In the vultures, which from their feeding on carrion do not require such strength of foot, we find a notable deviation from this general character, the toes becoming longer and slighter and the claws much weaker.