The pectoral arch in birds

The pectoral arch presents a long, narrow, and recurved scapula (Sc. Fig. 84 1/2), without any suprascapula; and a coracoid (Co.), fitted by its proximal end into the groove in the antero-lateral edge of the sternum. The inner ends of the coracoids occasionally overlap, as in Lacertilia; otherwise, the shoulder-girdle is unlike that of any of the Reptilia, except the Pterosauria. The coracoid is usually completely ossified, and presents no fontanelle. There is no distinct epicoracoid. The two bones take nearly equal shares in the formation of the glenoidal cavity, and usually remain unanchylosed and distinct in this region.

In the Ratitae the long axis of that part of the scapula which lies near the glenoid cavity is parallel or coincident with that of the coracoid, and the two bones become completely anchylosed. But, in all the Carinatae, the long axis of the scapula forms an acute, or only slightly obtuse angle (Ocydromus, Didus) with that of the coracoid. A small bone, the scapula accessoria, is developed on the outer side of the shoulder- joint in most Coracomorphae and Celeomorphae.

In the Carinatae, the glenoidal end of the scapula is divided into two portions; a glenoidal process, which expands to form the upper part of the glenoidal cavity, and to unite with the coracoid, and an acromial process, which gives attachment to the outer end of the clavicle. The glenoidal end of the coracoid is in like manner divided into two portions; a glenoidal process, which unites with the scapula, and a clavicular process, which articulates with the outer surface of the clavicle, near its outer end.

The clavicular process of the coracoid probably represents the procoracoid of Lacertilia. In the Retitae there is no distinct clavicular process, but the anterior part of the coracoid, near the glenoid cavity, may be produced and separated by a notch, or fontanelle, from the rest, as a lacertilian procoracoid. There is no trace of clavicles in the Apteryx and in some Parrots. In the Emeu, and in sundry Carinatae (some Parrots and Owls), the clavicles remain distinct from one another, or connected only by fibrous tissue; but, in the majority of birds, they are very early anchylosed together, and with the representative of the interclavicle, in the middle line, into a single bone, the furculum, the strength of which bears a pretty close relation to the exertion required of the wings in flight, or in natation. In the passerine birds the scapular end of the clavicle is usually expanded, and ossifies separately, as an epicleidium. A median process (hypocleidium) is frequently developed from the interclavicular part of the furculum, and this may be united with the carina of the sternum by strong fibrous tissue, or even by continuous ossification. In Opisthocomus, the furculum is anchylosed with the manubrial part of the sternum, on the one hand, and with the coracoids on the other. Anchylosis of the furculum with the coracoids has also been observed in Didus.