OSSEOUS SYSTEM OF AVES. 75 



The primary quill-feathers being the chief direct mechanical 

 instrument in the displacement of the air, the segment of the 

 limb supporting them is the longest and strongest in the most 

 powerful flyers, e.g. Swifts and Hummers, in which the pri- 

 maries are proportionally longer and stronger than in other birds : 

 but the various habits, habitats, and food of the feathered tribes 

 are associated with different kinds of aerial motion and call for 

 corresponding modifications of the instrument : thus the Frigate 

 and Tropic birds. Albatrosses, Terns, and other ablest flyers 

 among the Natatores, contrast strangely with the above-cited 

 Volitores in the proportionate length of the brachial and anti- 

 brachial segments of the pectoral limb : whilst the powerful 

 Kaptorial flyers show an intermediate more harmoniously ba- 

 lanced proportion of the several segments. All these are rela- 

 tively short and feeble in the heavier land birds which take but 

 brief and occasional flights ; and, as circumstances have rendered 

 this exertion less and less necessary, so the wings and their frame- 

 work have wasted away to the diminutive rudiments in the 

 Apteryx, and to zero in Dinornis. 



§ 130. Bones of the pelvic limb. — The segments of this limb 

 do not wholly correspond with those of the pectoral one, the 

 tarsus being absent or blended with the tibia or the metatarsus, 

 which immediately succeeds it. 



The femur, fig. 34, 65, has a cylindrical shaft, which, when not 

 straight, is slightly bent forward : it nearly equals the pelvis in 

 length in the Apteryx and some Ground-cuckoos {Geococcyx), 

 but is usually shorter ; it is very short in Dinornis elephantopus ; 

 shortest of all and most bent in Colymbus : it is always shorter than 

 the tibia, but in a minor degree in most Easores, Scansores, Voli- 

 tores, Cantores, some Natatores ( Tachypetes), and the Apteryx. The 

 head is hemispherical, proportionally small, and largely scooped 

 out above for the round ligament which fills up the vacuity in the 

 acetabular wall : it is sessile, with its axis nearly at right angles 

 to that of the shaft ; the articular surface is continued upon the 

 upper end of the bone which expands as it recedes from the head, 

 and usually rises above its level to form a trochanterian ridge 

 extending from behind forward and there produced and continued 

 a short way down the shaft. The outer (fibular) side coextensive 

 with this ridge is rather flattened and impressed by insertion- 

 marks of muscles. Rarely is there, as in Aptornis, a trochanter 

 minor, situated a little below the head on the inner (tibial) 

 side of the bone, or represented by a round rough surface, more 

 anterior, as in Dinornis. Assuming its cylindrical or subcylin- 



