The absence of a keel is characteristic of the other and smaller group of Birds, made up of the Ostrich, Rhea, Emeu and Cassowary, Moa and Kiwi, which from the resemblance the sternum thus bears to a flat-bottomed boat (_ratis_) are known as _Ratitae_. Whether keeled or not, the breast-bone affords a surface of attachment to the principal muscles of the fore-limbs, and its anterior end supports the coracoids, as in Fig. 2. Various processes are in most cases developed on the sides of the sternum itself, behind its junction with the ribs, especially towards the {8}posterior portion, where they often take the form of prolongations, the extremities of which occasionally meet and enclose what are called _fenestrae_; but these are unimportant when compared with the features presented by the anterior part. 4. The Pectoral Arch, or Shoulder-Girdle, consists of three pairs of bones, the Coracoids, the Scapulae or Shoulder-blades, and the Clavicles or Collar-bones, the last two usually coalescing in the median line into a V-shaped or U-shaped Furcula (the well-known Merry-thought); but in some groups, as certain Parrots, the clavicles are practically absent, while in others, as several Owls, they do not unite. The furcula often ossifies firmly with the anterior portion of the keel, and in _Fregata_, _Didus_ and the _Ratitae_, the coracoids and scapulae are fused together. [Illustration: FIG. 3.–Skeleton of the Limbs and Tail of a Carinate Bird. (The skeleton of the body is indicated by dotted lines.) _F_, digits; _Fi_, fibula; _HW_, carpus; _MF_, tarsometatarsus; _MH_, carpometacarpus; _OA_, humerus; _OS_, femur; _Py_, pygostyle; _R_, coracoid; _Rd_, ulna; _Sch_, scapula; _St_, sternum, with its keel (_Cr_); _T_, tibiotarsus; _Ul_, radius; _Z_, _Z^1_, digits of foot. (From Wiedersheim.)] 5. The Anterior Limbs, or Wings, are composed of the Humerus, or upper arm-bone, the Ulna and Radius (making the fore-arm), the Carpus or wrist, the Metacarpus and Digits, corresponding with the hand and fingers. The first of the three metacarpals bears the Pollex, or thumb, with one or two {9}phalanges (joints); the second the Index, representing man's first finger, with two or three joints; the third a weak digit with only one phalanx, except in _Archaeopteryx_, where there are four. The _Casuarii_ and _Apteryges_ possess an index only, which in the _Sphenisci_ fuses with the pollex. The basal joint of this is the normal place of attachment of the "bastard wing" (_alula spuria_). _Archaeopteryx_ had claws on all its fingers, but in recent Birds they occur on the first two only, being functionless in the adult. Wing-spurs arise from the carpal and metacarpal bones.