336 



CLASSIFICATION- OF VERTEBRATES. 



are well developed, their proximal ends being usually fused, thus 

 giving rise to the furcula or 'wish-bone.' The 

 scapula is either at right angles to the coracoid 

 (carinate birds) or in the same straight line 

 with it (ratite birds). 



In the fore limb the distal elements show 

 great reduction, the carpals being reduced by 

 fusion, while the hand has but three digits in 

 the adult (four in the embryo), these appar- 

 ently 2, 3, and 4 of the normal hand. The 

 metacarpals usually show more or less com- 

 plete fusion. 



The pelvis is large and characteristic. Its 

 bones are fused in all except Arch&opteryx, 

 and the acetabulum is perforated. The ilium 

 is elongated in front, and is fused with all the 

 vertebrae of the synsacrum (p. 331). The pu- 

 bis l and ischia are directed backwards, and are 

 usually without symphysis. The distal end of 

 the ischium may fuse either with the ilium 

 (most birds) or with the pubis. 



In the hind limb the noticeable features are 

 the great reduction of the fibula, the fusion of 

 the proximal tarsals with the tibia (tibio-tarsus), 

 and the distal tarsals to the metatarsals (tar- 

 so-metatarsus), the ankle joint thus being in- 

 tertarsal. In the foot but four toes occur in 

 the adult (five in certain embryos) these be- 

 lt is interest- 



. 329. Devel- 

 opment of wing 

 .skeleton of tern 

 {Sterna wilsoni}, 

 after Leighton. A, 

 early; B, late. c, 

 carpales ; cl, claw ; 

 me, metacarpals ; 

 r, radius ; re, radi- 

 al e ; u, ulna ; ue, ul- 

 nare; II- V, digits. 



ing i, 2, 3, 4. 



ing that usually the phalanges 



increase in number from the 



first to the fifth toe in the 



order 2, 3, 4, 5, as in many 



lizards. 



1 The bone called pubis corresponds 

 to the postpubis of dinosaurs ; the pre- 

 pubis being represented wholly or in part 

 in the pectineal process which occurs in 

 many birds (Fig. 331). 



FlG. 330. Pelvis of 

 raven, from the side. ./, 

 acetabulum; //., ilium : IS, 

 ischium ; O, obturator foramen ; /*, 

 pubis. 



