phylum viii YERTEBRATA 261 



teeth, and dental papillae or a dental ridge sometimes occur in the jaws of 

 some modern embryos (parrot, ostrich), true teeth are invariably wanting 

 among existing species. 



The pectoral arch is firmly attached to the thorax, in conformity with the 

 mechanical requirements of flight. The long blade-like scapula has no ridge, 

 extends along the dorsal side of the thoracic ribs, and takes part with the 

 coracoid in the glenoid cavity for the head of the humerus. Penguins are 

 exceptional in having the scapula broadly expanded posteriorly. The coracoida 

 are stout and pillar-like, their function being to receive the downward pull of 

 the wing muscles during flight. The clavicles, which are usually united in a 

 forked bone (furcula), sometimes act as supports : and by their union with 

 the coracoids at the shoulder-joint, and with the sternal keel below in the 

 centre, tend to resist the thrust of the wing muscles in flight. In some birds 

 of powerful flight (pigeons, humming-birds), however, the clavicles are so weak 

 as to be of no service from a mechanical standpoint. Among Ratites and also 

 a few Carinates (toucans, parrots) the clavicles are rudimentary or wanting, 

 and never unite to form a furcula. The furcula of Carinates may anchvlose 

 either with the keel of the sternum (Steganopodes) or with the coracoids (Opis- 

 thocomus), and in the frigate birds with both at once. 



The humerus of Carinates is expanded at both ends, and provided at its 

 proximal extremity with a strong pre-axial delto-pectoral ridge for the attach- 

 ment of the pectoral muscles. Its articular head is vertically elongated, and 

 there is often a pneumatic foramen adjoining it on the inner side. At its 

 distal extremity is a prominent oblique condyle on the inner side of the palmar 

 aspect for articulation with the radius, but there are never any condylar 

 foramina. The humerus of flightless birds is degenerate, and sometimes absent 

 altogether, as in many Moas. In the fore-wing, which is generally longer 

 than the humerus, the ulna is more strongly developed than the radius, and 

 often exhibits a row of tubercles along its lower edge for the attachment of 

 the secondaries. The carpus of adult modern birds contains only two bones 

 (radiale and ulnare) ; a distal row, however, is indicated in embryos by two 

 separate cartilaginous elements, which later become fused with the metacarpals. 

 The latter are never more than three in number, are unequally developed, and 

 in existing Carinates are more or less completely fused. Metacarpal No. I is 

 much reduced, and bears one or more, rarely two, short phalanges for the 

 support of the so-called bastard wing [alula); the second metacarpal usually 

 bears two phalanges and the third, one. The first and second digits are some- 

 times clawed (Struthio, Rhea, Chxmna), and in Archaeopteryx all three terminate 

 in claws. 



The three elements of the pelvis are anchylosed (except in Archaeopteryx), 

 and usually unite with the svnsacrum. In water-birds this union takes place 

 somewhat slowly, and in penguins and the great auk not at all. The ilium 

 is elongate, and may, as in birds of prey, extend much further in front of the 

 acetabulum than behind it. Ischium and pubis are both directed backwards. 

 The pubes often remain free from the ilia, and never unite with one another 

 to form a symphysis except in the ostrich. Since the retroversion of the pubis 

 is proved by embryological researches to be a secondary modification, no 

 homology can exist between this and the post-pubis of Ornithopodous Dino- 

 saurs, and the processus Uiopectinealis (Fig. 365) must be regarded as a structure 

 peculiar to birds alone. 



