POSTERIOE EXTREMITY OF VERTEBRATA. 



489 



cartilage, which is more closely connected with the metatarsus, is 

 articulated to it ; while a cuboid is articulated to the fibula. Owing 

 to the independence of the fibula, we have here a peculiarity, 

 which is only seen again in the Mammalia. In the Saurii, the 

 tarsal bone developed out of four primary elements (Fig. 274, A ts) 

 has no sigus of its constituent parts even in the embryo. It is 

 immovably connected with the tibia and fibula, while the distal 

 bones of the tarsus (tl) are more or less 

 connected with the metatarsus. This 

 appears to have been most complete 

 in the fossil Saurii (Ornithoscelida). 



In these arrangements we may per- 

 ceive an outline of what obtains in the 

 foot of the Bird, which, in its em- 

 bryonic condition (Fig. 274, B), pre- 

 sents us with those characters which 

 are permanent in many Reptiles. The 

 fibula (f)) extends to the tarsus. This 

 is formed of two pieces of cartilage ; 

 the upper one (ts) is undoubtedly homo- 

 logous with the bone, which is made up 

 of four elements in the Eeptilia ; the 

 lower one (ti) corresponds to the distal 

 series of tarsal bones. The metatarsus 

 is made up of five bones, which were 

 primitively separate, but only four of 

 these (B, I — IV) carry toes, while the 

 fifth is very small and completely fused 

 with the distal portion of the tarsus. 

 The difference between the adult and 

 embryonic arrangements consists in 

 the degeneration of the fibula (Fig. 

 275, b'), which later on is attached to 

 the tibia, as a small appendage (¥), 

 and which never reaches the tarsus (b) . 

 The proximal tarsal cartilage fuses 

 with the tibia, and forms its articular 

 condyle; the distal one unites with 

 the single piece (c), which is formed 

 from the fusion of the three longer 

 metatarsal bones, and in which no 

 permanent signs of separation can be 



made out except what is implied by the separate condyles at its 

 distal end (Fig. 275, c'). The metatarsal of the hallux remains dis- 

 tinct, and generally forms a small appendage of the large tarso- 

 metatarsus. The arrangements, therefore, which are seen in the foot 

 of the Reptile are still further developed in that of the Bird, for the 

 parts which in the former are merely united firmly together, are fused 

 in the latter ; the foot still moves on the same intertarsal joint. 



Fig. 274. Skeleton of the foot of 

 aEeptile (Lizard) (A) and a Bird 

 (B) ; the latter is in its embryonic 

 condition. / Femur. t Tibia. 

 p Fibula. ts Upper, tl Lower 

 piece of the tarsus, m Metatarsus. 

 I — V Metatarsalia of the toes. 



