OSSEOUS SYSTEM OF AVES. 35 



to most birds. It does not take place in Ajiteryx, Dinornis, 

 Struthio, fig. 24, c\ The ilium forms an angular projection above 

 the posterior ischial junction in the Albatross, Skimmer, Duck, 

 fig. 22, n. Ibis, Spoon-bill, Woodcock, Pigeon, most Volitores 

 and Insessores. The principal pneumatic foramen of the ilium 

 is on the outer and under part of the post-acetabular division. 

 The ilium developes an oblong articular surface on the pro- 

 minence extending from the upper and back part of the aceta- 

 bulum. 



The ischium, fig. 24, 63, fig. 22, ^, m, fig. 23, c, i, is a long, nar- 

 row, flattened bone ; thickest where it forms the back part of the 

 acetabulum, becoming thinner and broader as it extends back- 

 ward, with the lower border turned slightly outward; generally 

 placed parallel with its fellow, but diverging in the Ostrich ; of 

 nearly uniform breadth in this wingless bird, fig. 24, and in 

 the Apteryx, but usually expanding to its hinder end, and 

 there coalescing with the ilium. Just beyond the acetabular 

 part the ischium contracts, presenting a smooth and thick upper 

 border to the ischiadic notch or foramen, fig. 22, Z, fig. 23, h, and 

 a similar lower border to the foramen or notch, ib. o, fig. 24, /, 

 which transmits tlie tendon of the obturator internus muscle ; it 

 then becomes lamelliform, with thin margins, usually increasing 

 in depth, and often bent down at its termination to join the 

 pubis, and circumscribe, as in fig. 24, the obturator foramen, o. 

 In the Ostrich, the ischium does not join the ilium posteriorly, 

 and the ischiadic notch remains open; its coalescence with the 

 ilium, beyond the ischiadic foramen, is usually extensive, as in 

 figs. 22 and 23. 



The most singular modification of the ischia is seen in the 

 Rhea, in which they meet below the sacrum and coalesce with 

 each other for some extent, almost obliterating here the bodies of 

 the sacral vertebra?. 



The pubic bones, fig. 21, /?, fig. 24, 64, present an analogous 

 exceptional condition in another member of the Cursores, viz. 

 the Ostrich, in which they unite together at their hind ends, 

 forming a ^ symphysis,' which is curved downward and forward, 

 fig. 24, h ; in Gyps fulvus the same ends curve toward and 

 almost touch each other. In other birds the pubic bones are 

 directed backward, with usually a curve convex outward, and 

 terminate freely, or are united to the ischium above, as in 

 fig. 34, h, the pelvis being thus an open one, as a rule, in 

 Birds. The pubis forms the lower and front portion of the 

 acetabulum, beyond which it quickly contracts, exchanging its 



D 2 



