64 DESCRIPTIONS OF PREPARATIONS. 



band of tissue continuous with the clavicles. It therefore represents in part the 

 interclavicle of a Reptile. The anterior extremity of the band forms the hypo- 

 cleidium and the interclavicular or sterno-clavicular ligament. Their view has 

 been recently controverted by Miss B. Lindsay, who contends that the Avian 

 sternum consists (i) of a costal sternum derived from the ribs ; (2) of a meta- 

 sternum, apparently a growth from (i) ; (3) of anterior lateral processes (costal 

 processes), either outgrowths of (i) e.g. Struthio, or formed from anterior ribs, 

 e.g. Chick; (4) of a keel, an outgrowth of (2); and (5) posterior lateral (i.e. 

 xiphisternal) processes derived from (2) also ; together with other structures some- 

 times present. The subject requires re-investigation. The shape, disposition, &c. 

 of the several parts of the sternum vary much in Birds. 



The scapula and coracoid are fused in Ratitae and connected by ligament 

 in Carinatae. Each ossifies from a single centre. With rare exceptions (Psopkia 

 among Carinatae, Apteryx among Ratitae), the inner or anterior border of the 

 coracoid becomes partially ligamentous. The sub-scapular process when large, e. g. 

 Gull, Eagle, is pierced by a foramen ; when small, this foramen lies in the ligament. 

 A foramen similarly placed exists in the complete coracoid of Psophia and Apteryx, 

 and in the Crocodile and most Lizards. It transmits a nerve for a muscle, the 

 second pectoral in the Bird. These facts point to a homology, as maintained by 

 Sabatier, of the sub-scapular process of the coracoid with the praecoracoid of the 

 Reptile, i. e. with its proximal extremity. In the embryo, as figured by Miss Lindsay, 

 there appears to be a large praecoracoid rudiment. The ligamentous portion 

 of the coracoid is well characterized by its tough nature and parallel fibrillation. 

 It is invaded more or less by ossification in Ratitae, and in an old Ostrich its 

 anterior margin ossifies, inclosing a coracoid fontanelle. In a Carinate it forms 

 part of the coraco-clavicular ligament. The clavicular process of the coracoid gives 

 attachment in Carinatae to the deltoideus minor muscle and principal ligament of 

 the shoulder-joint. In Ratitae it is represented by a mere roughness or slight 

 tuberosity. It must be considered as a process developed for the same reason and 

 for the same purpose as the deltoid tubercle or supinator ridge in the humerus of 

 some Mammalia, e.g. Armadillo. The structures attached to it are of prime 

 importance in flight. The two clavicles ossify parosteally. They may be absent, as 

 in all Ratitae save the Emu, and in some Parrots ; or fail to meet ventrally Emu, 

 Toucan, some Parrots and Owls. The upper end (epicleidium), small in the Pigeon, 

 may be large, e. g. Goose, and is stated to be in this case cartilaginous in the embryo. 

 The hypocleidium is small in the Pigeon ; it is large and directed downwards, e. g. 

 Fowl, or backwards, e.g. Rook. A coraco-clavicular ligament unites each clavicle 

 to the inner border of the coracoid, and a sterno-clavicular ligament unites the hypo- 

 cleidium to the carina. Irregular ossifications may appear in these membranes. 



The proportions of the segments of the fore-limb one to the other vary much. 

 In Ratitae they show scarcely a trace of the characteristic Avian position when at 

 rest. The Cretaceous Hesperornis has only the humerus, and the limb is either 

 absent or reduced in the extinct Dinornithidae. In Archaeopteryx the parts of the 

 hand are free, and each digit appears to have borne a claw. A carpo-metacarpus 

 exists in all other birds. In the embryo fowl a mass of cartilage (=carpalia i + ii) 

 corresponds to the two first metacarpals, and a second (=carpalia iii + iv) to the 

 third and embryonic fourth metacarpals. Uria grylle has, according to Morse, 

 embryonic claws to the first and third digits. 



