Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/740

722 a less degree, in the Great Auk, the humerus becomes flattened from side to side, the proximal end is singularly modified, and at the narrow distal end the articular sur face for the radius lies completely in front of, and rather above, that for the ulna. The ulna, which often presents a series of tubercles, indicating the attachment of the secondary quill feathers, is usually a stronger and a longer bone than the radius. There are only tvo carpal bones, one radial and one ulnar. There is one exception to this, namely, in the Screamer (Chauna chavaria), which has three carpals on the left side, the lower arcuate bone having two representatives. In the Apterygidae and in the Casuariidce there is but one complete digit in the manus. It appears to answer to the second of the pentadactyle limb, and is provided with a claw. In the Struthionidce and Rheidce, and in all the Carinatce, there are three digits in the manus, which answer to the pollex and the second and third digits of the pentadactyle fore-limb ; and the metacarpal bones of these digits are ankylosed together. As a rule the meta carpal of the pollex is much shorter than the other two ; that of the second digit is strong and straight ; that of the third is more slender and bowed, so as to leave an inter space between itself and the second, which is often filled up by bony matter. The pollex has two phalanges, and the second of them is, in many birds Rhea, the Screamer, &c. pointed, curved, and ensheathecl in a horny claw. The second digit has two and sometimes three phalanges, as in the Swan ; and the terminal phalanx is similarly pro vided with a claw in sundry birds, e.g., the Swan and Rhea. In the Ostrich both the pollex and the second digit are unguiculate. The third digit possesses one phalanx, besides it s ankylosed metacarpal, and is always devoid of a claw. It is a singular circumstance that the relative proportions of the humerus and the manus should present the most marked contrast in two groups of birds which are alike remarkable for their powers of flight. These are the Swifts and Humming-birds, in which the humerus is short and the manus long, and the Albatrosses, in which the humerus is long and the manus relatively short. In the Penguins the pollex has two free phalanges, and its metacarpal bone (which is distinct in the young birds) ankyloses with that of the second digit. The third metacarpal is slender and straight. The bones of the nianus are singularly elongated and flattened. 14. Pelvis and caudal vertebra of adult Fowl, side view, natural size. ., ilium ; is., ischium ; pb., pubis; d.l, dorso-lumbar vertebrae : cd., caudal vertebrae ; am., acetabulum. {{ti|1em|The pelvis of a Bird (fig. 34) is remarkable for the great elongation, both anteriorly and posteriorly, of the iliac bones (il.), which unite with the whole length of the edges of the sacrum, and even extend forwards over the posterior ribs of the dorsal region. Below, each iliac bone forms a wide arch over the acetabulum (am.), the centre of which is always closed by fibrous tissue, so that in the dry skeleton the bottom of the acetabulum is always per forated by a wide foramen. An articular surface on the ilium, on which the great trochanter of the femur plays, is called the antitrochantcr. In all ordinary birds the ischium (is.), which broadens towards its hinder end, extends back nearly parallel with the hinder part of the ilium, and is united with it by ossification posteriorly. The ischiosciatic interval is thus converted into a foramen. The pubis (pi.) enters by its dorsal or acetabular end into the formation of the acetabulum, and then passes backwards and downwards as a comparatively slender, curved bone, nearly parallel with the ischium. It is united with its fellow only by fibrous tissue. Very few birds present any important variation from this structure of the pelvis. In Tinamus, Casuarius, Dromceus, Apttryx, Dinornis, the ischium is not united with the backward extension of the ilium by bone. In Rhea the ischia unite with one another beneath the vertebral column ; and the verte brae in this region, that is, from the true sacral to the end of the iliac roof, become undistinguishable, being formed into a long slender uro-sacral style. In Struthio alone, among Birds, do the pubes unite in a median ventral symphysis (see Mivart, T. Z. &amp;lt;S., vol. i. part 7, pp. 434, 435, figs. 72, 73). Another not less remarkable circum stance in the Ostrich is that the 31st to the 35th vertebra; inclusively (counting from the atlas) develop five lateral tuberosities. The three middle tuberosities are large, and abut against the pubis and the ischium. In these vertebra}, as in the dorsal vertebrae of the Chdonia, the neural arch of each vertebra shifts forward, so that half its base arti culates with the centrum of the next vertebra in front, and the tuberosities in question are outgrowths, partly of the neural arch, and partly of the juxtaposed vertebral centra between which it is wedged. Hence in young Ostriches the face of each tuberosity exhibits a triradiate suture. A small bone is sometimes found on the ventral edge of the pubis, at its middle ; this has been supposed to repre sent a marsupial lone (Garrod, P. Z. S., Mar. 1872, p. 359).}} The upper articular head of the femur is rounded, and its axis is almost at right angles with the body of the bone ; a structure which is not found in ordinary Reptilia, but exists in the Iguanodon and other Ornithosce- lida. The shaft is re latively short and thick, and the two terminal condyles are large and elongated antero-poste- riorly. A prominent ridge, which plays be tween the proximal ends of the tibia and fibula, is apparent upon the posterior and inferior surface of the outer condyle. A similar ridge is faintly developed in some Lacertilia, and is well marked in the Dinosaurian Eeptiles. A patella is usually present, but it is some times absent, and may be double. The fibula of Birds (fig. 36, F.) is always imperfect, 