. Elements of Comparative Anatomy. 490 COMPAEATIVE With regard to the toes, we find five to be the dominant number in the Ecptilia ; it is in Birds only that they fall to four, or three, or even to two (Struthio). The phalanges of the toes generally increase in number from within outwards; there are two on the hallux and five on the fourth toe. This holds for the Saurii, Crocodilini, and Aves. There are not so many in the Amphibia or Chelonii. Amongst the Eeptilia the limbs are reduced in the snake-like Lizards, and in all Ophidii, among which the Peropoda only are provided with any
. Elements of Comparative Anatomy. 490 COMPAEATIVE With regard to the toes, we find five to be the dominant number in the Ecptilia ; it is in Birds only that they fall to four, or three, or even to two (Struthio). The phalanges of the toes generally increase in number from within outwards; there are two on the hallux and five on the fourth toe. This holds for the Saurii, Crocodilini, and Aves. There are not so many in the Amphibia or Chelonii. Amongst the Eeptilia the limbs are reduced in the snake-like Lizards, and in all Ophidii, among which the Peropoda only are provided with any rudiments of them at all. Gegenbaur, C, Uiitersucliungen zur vergleich. Anat. I. Leipzig, 1864. § 369. The special differentiations in the skeleton of the hind-limb of Birds and "Reptiles do not re- semble those which are seen in the MammaHa. As a rule it is less altered than the fore-limb. In the Perissodactyla, many Rodents, etc., the femur is distinguished by the possession of a third trochanter. The tibia is the most im- portant bone of the leg; the fibula is often rudi- mentary, especially in the Ungulata. In the Artiodactyla the distal end remains; it is articu- lated to the tibia and to the tarsus (astragalus), and appears to enter into the composition of the latter. In some (as in Rodents and Insectivora) the tibia and fibula are complete, and are fused together. The tarsus is the most characteristic part; it is attached by two pieces to the leg, but, as a rule, only one of these forms the ankle- joint. The process on the second bone (calcaneum), of which there were indications in the Crocodilini, is still more developed. The centrale remains separate, but passes to the inner edge of the foot, where it forms the navicular. In some of the Prosimige it unites with the calcaneum to form a long process (Macrotarsi). Of the five distal bones the two outer ones are always replaced by the cuboid, while the three inner ones generally remain distinct (cunei- form). When th
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