51 J 



Comparative Morphology of Chordates 



quadrupeds. The tibia is thick and strong, but the fibula is reduced 

 to a splint which is largely fused with the tibia. The proximal tarsals 

 (Fig. 402Z?) become solidly fused to one another and to the distal end 

 of the tibia, thus forming a tibio-tarsus (Fig. 402A). The meta- 

 tarsals are much elongated and, except the very small first, are fused 



Fig. 401. Bird skeleton. (C, C). Carpals; (CI) 

 clavicle; (Co) coracoid; (F) fibula; (Fe) femur; (H) 

 humerus; (J I) ilium; (Js) ischium; (Mc) metacarpals 

 (carpo-metacarpus) ; (P', P", P'") phalanges of digits 

 1-3; (Pb) pubis; (Pu) uncinate process of rib; (R) 

 radius; (Rh) cervical ribs; (S r ) sternal rib; (Sc) scapula; 

 (St) sternum; (T) tibia; (7m) tarso-metatarsus; (U) 

 ulna; (Z) Phalanges of toes. (Courtesy, Zittel: "Hand- 

 buch der Palaeontologie," Munich, R. Oldenbourg.) 



together. To the proximal end of the resulting bone the distal tarsals 

 are solidly fused, thus forming a complex bone, the tarso-metatarsus. 

 Therefore, in addition to the usual thigh and shank, there is a moder- 

 ately long tarso-metatarsal segment, and the set of long toes makes a 

 fourth segment. The joint between tibio-tarsus and tarso-metatarsus 

 evidently does not correspond to the usual tetrapod ankle-joint, which 

 is between tibia (and fibula) and the tarsal region. In many reptiles 



