In correlation with their habits of jumping, the posterior limbs in Anura are much lengthened and considerably modified. The tibia and fibula are completely fused. The intermedium is absent, while the tibiale and fibulare are greatly elongated. Tarsalia 4 and 5 are absent. Five digits are always present, and there is a prehallux formed of two or more segments. In general the posterior limbs in Labyrinthodontia bear the closest resemblance to the anterior limbs; in some cases three centralia are found. In Ichthyoidea, and in most Labyrinthodontia, the cartilages of the carpus and tarsus remain unossified; in Salamandrina and in Anura they are generally ossified. FOOTNOTES: [59] O. Hertwig. Ueber das Zahnsystem der Amphibien. _Arch. mikr. Anat._ supplem. Bd. XI. 1875. [60] G.A. Boulenger, _P.Z.S._ 1890, p. 664. [61] See p. 14. [62] See many papers by W.K. Parker published in the _Phil. Trans._ of the Royal Soc. [63] Perhaps this bone includes supra-orbital and postorbital elements. [64] The first digit present is sometimes regarded as the pollex, but from analogy with Anura it is probable that the pollex is the missing digit. CHAPTER XIII. SAUROPSIDA. This great group includes the Reptiles and Birds and forms the second of the three into which the Gnathostomata may be divided. There is nearly always a strongly-developed epiblastic exoskeleton which has the form of scales or feathers, and in some cases a dermal exoskeleton is also well developed. In living forms the notochord never persists, being replaced by vertebrae, but in some extinct forms the centra are notochordal. The vertebral centra are ossified, and only in exceptionally rare cases have terminal epiphyses. The skull is well ossified and has membrane bones incorporated in its walls.