The skeleton of the Tortoise is, perhaps, the most extraordinary structure with which we are acquainted. This oddly-organised animal when first seen strikes the beholder with astonishment. The carapace and plastron, with their connecting plates, form a sort of protecting box, in which the animal lives, its head and tail excepted. In the land Turtles the head and feet, which are comparatively senseless, can be withdrawn within the protecting armour. The ribs and sternum are both placed quite on the exterior of the body, so as to form a broad dorsal shield on the upper surface, and an equally strong ventral plate; between these, the limbs and the head can be more or less completely retracted. Nevertheless, the modifications in the arrangement of the elements by which these changes are accomplished are of the simplest nature. In the common Tortoise, _Emys Europæus_, the vertebræ of the neck and tail being connected together in the ordinary manner, the neck and caudal region of the spine present their usual flexibility, but the dorsal vertebræ are strangely distorted, the upper arch being disproportionately developed, while the bodies remain almost in a rudimentary state; the spinous processes of these vertebræ are flattened and converted into broad osseous plates, which form a longitudinal series along the centre of the back, and connected together by means of sutures. The ribs are changed into broad flat bones firmly united by sutures to each other, and also to the lateral margins of the spinous processes of the vertebræ, so that they form together a single broad plate; the heads of the ribs are feebly developed, and the intervals between them and the bodies of the vertebræ filled up with ligament. The margin of the shield thus formed by the dorsal ribs is further enlarged by a third set of flat bones fixed by sutures around the whole circumference of the carapace. The plastron, or ventral plate, is made up of nine pieces, of which eight are arranged in pairs; but the ninth, which is always placed between the four pieces composing the two anterior pairs, is single, and occupies the mesial line. The bones of the shoulder and hip are placed within the thorax, and articulated to the sides of the vertebral column. Of this vertebral column in these extraordinary animals, Professor Owen remarks that the manifold modifications of the framework which render it a portable abode, appear to have been given as a compensation for inferior powers of locomotion, and the absence of offensive weapons. But with all its modifications, the same number of pieces are found in the bony skeleton as in other ordinary vertebratæ, the form and volume of many of these pieces being alone changed.