LIMBS. 107 



other points the humerus, radius, and ulna correspond closely with those of 

 existing Birds. There was only one carpal (radiale), and the manus consisted 

 of three free metacarpals and digits, of which the first possessed two, the 

 second three, and the third four phalanges : all the digits were provided 

 with claws (Fig. 92). 



Ten families of existing Carinate Birds possess the same number (two) of 

 phalanges on the first finger as Archseopteryx, the distal one hearing a claw. 

 Four families of Carinatae also possess three phalanges on the second finger, 

 like Archaaopteryx, but in only two of these families is there a claw on this 

 digit. The third finger in all existing Carinates has only one phalanx, 1 as 

 compared with four in Archaeopteryx, and this never bears a claw. Amongst 

 the Ratitae, Apteryx and Casuarius possess only a single digit (the second), 

 and it, like the first finger of Struthio and Khea, is provided with a claw. 



The strongly-developed and pneumatic 2 bones of the arm and 

 fore-arm stand out in sharp contrast with the greatly reduced 

 skeleton of the manus ; and the anterior extremities in most Birds 

 of flight, as the principal organs of locomotion, greatly exceed the 

 posterior in size (Fig. 93, H, R, ?7). 



FIG. 93. ANTERIOR EXTREMITY OF BLACKBIRD (Turdus merula). 



H, humerus ; R, radius ; U, ulna ; r, radiale ; u, ulnare ; Me, Me, the three meta- 

 carpals, with which the distal row of carpals has united ; / to ///, the three 

 digits. 



The tarsus of Birds consists in the embryo of three elements, 

 two small proximal and a broader distal. The former (tibiale 

 and fibulare) unite later with the distal end of the tibia, thus 

 forming a tibio- tarsus, while the latter, which corresponds to 

 tarsalia 1 to V, becomes included in the base of the metatarsus. 

 Thus the foot of adult Birds no longer possesses any distinct tarsal 

 elements, though, as in Chelonians and Lizards, the foot really 

 moves by an intertarsal articulation. Of the original five meta- 

 tarsals, the fifth soon disappears, while the second, third, and fourth 

 become united with one another and with the distal element of 

 the tarsus to form a single bone, the tarso-metatarsus (Fig. 

 94, Mt). The first metatarsal remains to a greater or less extent 

 independent. 



1 In embryos of the Duck a cartilaginous rudiment of a second phalanx is pre- 

 sent in the third finger of the manus. The Ostrich alone amongst existin^ Birds 

 possesses a second phalanx on the third finger. 



2 See p. 262, concerning the pneumatic character of the bones. 



