In most reptiles there are two pairs of pentedactylate limbs provided with claws, but in nearly all Ophidia and some Lacertilia (_Amphisbaena_, _Lialis_, _Anguis_) the limbs have entirely disappeared. In a few Ophidia such as _Python_ traces of the posterior limbs occur, and in _Chirotes_ among the Amphisbaenidae there are minute anterior limbs. The Lacertilians, _Chalcides_ (_Seps_) and _Ophisaurus_ (_Bipes_, _Pseudopus_) have very small posterior limbs. The limbs are as a rule adapted for walking, but in Ichthyosauria, Sauropterygia, Pythonomorpha and some Chelonia, they have the form of swimming paddles, the relative size of the manus and pes being increased, while that of the proximal and middle portions of the limbs is reduced. This reduction is carried to its furthest extent in the Ichthyosauria in which radius and ulna, tibia and fibula, have the form of short polygonal bones similar to those constituting the manus and pes. In the Pythonomorpha the reduction of the limb bones is not quite so marked, in the Sauropterygia it is less, and still less in the Chelonia. In the earlier Ichthyosauria too, the limb bones are not so short as they are in the later forms. The Ichthyosaurian limb is also remarkable, firstly for the fact that both humerus and femur are terminated by concave articulating surfaces instead of by convex condyles, and secondly for the great multiplication of the phalangeal bones, each digit being sometimes composed of a series of over twenty. Sometimes too the number of series is increased, either by the bifurcation of some of the digits or by the development of marginal bones. In the Sauropterygia the phalanges are likewise increased above the normal but not so much as in Ichthyosauria. The humerus and femur of Sauropterygia are noticeable for the enormous size of the terminal epiphyses which form in each case by far the greater part of the bone. THE ANTERIOR LIMB. The anterior limb is usually approximately equal in length to the posterior, but in many Dinosauria it is considerably the shorter of the two. The humerus is generally without distinct condyles, but they are well developed in the Theromorpha, the Lacertilia and _Sphenodon_. In the Theromorpha, some Rhynchocephalia, and some Sauropterygia, such as _Mesosaurus_, the humerus has an ent-epicondylar foramen; in Lacertilia, Chelonia and some Dinosauria there is an ect-epicondylar foramen or groove; _Sphenodon_ possesses both ent- and ect-epicondylar foramina. The radius and ulna are always separate. In some Chelonia, such as _Chelydra_, the carpus has a very simple arrangement, namely, a proximal row of three bones, the radiale, intermedium and ulnare, and a distal row of five carpalia, with one bone, the centrale, between the two rows. Many reptiles have a carpus only slightly different from this. Thus the carpus in _Sphenodon_ differs mainly in having two centralia, that of most Lacertilia, in having the centrale and intermedium fused.