A typical vertebra consists of a centrum, and an arch, with articular surfaces for two ribs, and is called heterocoelous when the facets, or connecting surfaces, are saddle-shaped, a condition characteristic of, and restricted to, Birds. It is amphicoelous, or biconcave, when each end is hollowed, as in the dorsal region of _Ichthyornis_ and probably in _Archaeopteryx_; procoelous, when concave in front (as is common in Reptiles); opisthocoelous when concave behind (as in many Mammals). 2. The Ribs are doubly attached to the vertebrae by a head (_capitulum_) and a knob (_tuberculum_); and have a neck, a dorsal, and a ventral portion, each dorsal section (save on the last rib) possessing an "uncinate process" or thin, bony posterior projection, except in _Archaeopteryx_ and the _Palamedeidae_. Should the ventral piece articulate with the sternum the rib is "true," otherwise it is called "false"; moreover the cervical and frequently the post-thoracic ribs are fused with the cervical vertebrae and the ilia respectively. [Illustration: FIG. 1.–Third cervical vertebra of Woodpecker (_Picus viridis_). (Viewed anteriorly.) _Ft_, vertebrarterial foramen; _Ob_, upper arch; _Pa_, articular process; _Psi_, haemal spine; _Pt_, _Pt_, the two bars of the transverse process, shewn on one side ancylosed with the cervical rib (_R_); _Sa_, articular surface of centrum. (From Wiedersheim.)] 3. The Breast-bone (_Sternum_) presents two different styles–according to whether it exhibits on its ventral surface a median ridge or keel (_carina_), or not. In the former case, which is that of by far the greater number of existing Birds (hence termed _Carinatae_), the keel is of variable size, being correlated with the power of flight. It is exceedingly deep in the Swifts, Humming Birds, and certain Petrels, but dwindles almost to disappearance in some flightless forms such as the Dodo, the Kakapo (_Stringops_), the extinct New Zealand Goose (_Cnemiornis_), and a good many Rails. {7}[Illustration: FIG. 2.–Skeleton of the trunk of a Falcon. _Ca_, coracoid, which articulates with the sternum (_St_) at †; _Cr_, keel of sternum; _Fu_ (_Cl_), furcula (clavicles); _G_, glenoid cavity for humerus; _S_, scapula; _Un_, uncinate process; _V_, vertebral, and _Sp_, sternal, portion of rib. (From Wiedersheim.)]