GENERAL STRUCTURE. 121 5 



the clavicles are rudimentary or wanting, and never unite to form a 

 furcula. No Bird preserves a distinct interclavicle, although it 

 has been thought that the median portion of the furcula represents 

 this element — a view which, from developmental considerations, is 

 now regarded as improbable. The furcula itself may anchylose 

 with either the manubrium of the sternum, or with the coracoids ; 

 and very rarely both these unions occur together. 



In the pectoral limb the component bones, when at rest, have 

 been greatly altered from their original primitive position ; the 

 change being somewhat greater than that occurring in the majority 

 of Mammals. The humerus of Carinates (fig. 1106, h) is expanded 

 at both ends ; the proximal extremity having a strong preaxial delto- 

 pectoral ridge for the pectoral muscle. The pneumatic foramen 

 occurs on the proper dorsal aspect ; and at the distal extremity the 

 prominent oblique condyle, on the outer side of the palmar aspect 

 for the articulation of the radius, forms a very characteristic feature. 

 In the Ratitse this bone has less distinctive characters, and may be 

 very small, or even absent. There is no foramen for vessels at the 

 distal extremity of the humerus in any member of the class. The 

 radius and ulna (fig. n 06, 11, r) are always separate; the former, 

 which is usually the larger of the two, being frequently marked with 

 a line of tubercles for the attachment of the secondary wing-feathers. 

 In the adult of recent Birds the free bones of the carpus are reduced 

 to two — a radiale and ulnare ; but in the Jurassic Archceopteryx only 

 the radiale remains. In the latter genus the manus comprises three 

 free metacarpals and digits ; of which the first carried two, the 

 second three, and the third four phalangeals ; the terminal bone in 

 each digit being furnished with a claw. In existing Carinate Birds 

 the three metacarpals (fig. 1106) are more or less completely fused 

 together ; and, according to Professor Weinsheimer, ten families 

 possess the same number (two) of phalangeals in the first digit as 

 in Archceopteryx, the distal one bearing a claw ; while four families 

 also possess three phalangeals in the second digit, like Archceopteryx, 

 but in only two of these families is the terminal phalangeal provided 

 with a claw. In all existing Carinates the third digit has only a 

 single phalangeal, without a claw. Among the Ratitse there are 

 three digits in Struthio and Rhea ; but Apteryx and Casuarius have 

 but a single digit, which is clawed. There is usually an interspace 

 between the curved second and third metacarpals which may be 

 filled up by a thin plate of bone. 



The characters of the pelvis have been already briefly alluded to. 

 The ilium (fig. 1107) is always produced considerably on both sides 

 of the acetabulum, and in some cases, as in the Apteryx, the anterior 

 production is very great ; it articulates with the long sacrum, of 

 which the homology has been already noticed. The ilium arches 



