_Reptiles._—In the class of reptiles the characters afforded by the form of the extremities are not less important. Of the four orders into which this class is divided, one—that of the serpents (_Ophidia_)—is altogether destitute of members, with the exception of the rudimentary traces of extremities met with in some few species (_Anguis_, _Bimanes_, and _Seps_). In another order, that of amphibious animals (_Batrachia_), as the frogs, toads, &c., the extremities are still typical of certain peculiarities of organisation. Some of these creatures may be said to remain in an imperfectly developed or embryonic condition during their whole existence. Instead of breathing in adult life by means of lungs, as do the more perfectly organised of the _batrachia_, they retain throughout existence that peculiar structure of the respiratory apparatus—the branchial tufts—which in the latter are present but in the embryo condition. In those of the amphibious animals which breathe by means of permanent branchia (_Perenni-branchiata_), such as the _Lepidosiren_ and the _Proteus_, the extremities are either mere filamentary appendages, or at most but very imperfectly-formed organs, and always typical of their comparatively inferior organisation; while, in the higher species, that is, those provided with lungs, and losing their branchiæ in the adult state (_Caduci-branchiata_), as the frog, the salamander, &c., the extremities are well formed and perfectly developed. The third order of reptiles—that of the lizard tribe (_Sauria_)—is made up of members very dissimilar both in their appearance and general habits. In the aquatic crocodiles the toes are connected by means of a membrane; like ducks and other aquatic animals, these creatures are web-footed. The alligator, so nearly allied to the crocodile in structure and habits, with the exception that it is more terrestrial in its nature, is furnished with a foot the toes of which are only webbed in half their length, the other half being free and unconnected by membrane, their amphibious mode of life being indicated by their foot. If we compare the foot of these creatures with that of the true lizards, the difference of structure will be found to be considerable. The foot of the true lizard is provided with five toes to both the fore and hind members, while attached to the hinder members of crocodiles and alligators we find but four toes. The toes of the lizard are not connected by a membrane, but are separate, and armed with nails, for these creatures live on land, and have need of well-constructed locomotive organs to enable them to catch their active prey. Among lizards, that remarkable animal the chameleon, that is provided with a particular structure of the skin, whereby its change of colour is effected, is distinguished by the possession of a member, the structure of which is not less peculiar. The toes are arranged in two packets, which are capable of being brought into opposition—a