34 ANATOMY OF VERTEBRATES. 



in the proportions of the pre-acetabular and post-acetabular exten- 

 sions, and in the degree of divergence of the latter from the sacrum. 

 The longest and narrowest ilia are seen in certain Natatores (Po- 

 diceps, Colymhus, fig. 34, a, d, Uria) and in Cursores {^Struthio, 

 fig. 24, h, c , Dromaius) : the shortest and broadest ilia are seen 

 in certain Volitores, Scansores, and Insessores. In the Grebe and 

 Loon the ilia unite with the summits of the sacral spines behind 

 the acetabula, and diverge for a broader interposed neural expan- 

 sion anteriorly : in most birds the divergence is shown at the post- 

 acetabular portions, as in fig. 22, g, g^ the pre-acetabular plates 

 d, d, converging to the summits of the sacral spines, ib. h. A few 

 birds {Podargus^ Tachypetes) retain the extent of sacral interpo- 

 sition which obtains at an early stage of pelvic developement in all 

 birds. ^ In the old Apteryx the ilia almost meet along the summit 

 of the sacral ridge to mthin a short distance of their hind end, 

 where an epiphysial piece of bone is sometimes found wedged 

 between this end and the anterior caudal vertebrae. The anterior 

 border of the ilium is usually more or less convex : in Tinamus, 

 Crax, Ojiocrotalus, it is almost straight : in Geococcyx, Corythaix, 

 Scolephagus, it is emarginate or concave, the external angle being 

 produced outw^ard : in Limosa it is angular ; the point being 

 formed by the commencement of the ^gluteal ridge:' this, which 

 is well-marked in most birds, describes a curve, concave down- 

 ward, and terminates above or behind the acetabulum, as atjf, 

 fig. 22, marking off the post-acetabular convex part of the ilium, 

 g, h. This part is the longest in Grebes, Loons, fig. 34, d, and 

 the Ostrich, fig. 24, c' : it is the shorter division in Petrels, Gulls, 

 Cranes, and most smaller Grallatoi-es, in the Apteryx^ in most 

 Insessores, and especially in diurnal Raptores, fig. 23, g, i : in 

 many birds it forms half the length of the ilium. In some birds 

 ( Cursores) it is narrower than the fore part of the ilium ; in 

 others, especially Geococcyx, it is broader : in most the breadth is 

 about equal, although the ilium may seem broadest behind from 

 its coalescence with the horizontal expansions from the sacral 

 spines. The upper is divided from the outer surface of the post- 

 acetabular part of the ilium by a prominent ridge in most birds, 

 fig. 22, g, which generally overhangs the outer surface ; in 

 Geococcyx to a remarkable extent, like a wide pent-house, pro- 

 ducing a deep concavity in the outer and back part of the ilium 

 where it coalesces with the ischium. This coalescence, converting 

 the ischiadic notch into a foramen, fig. 22, I, fig. 23, h, is common 



' XV. p. 45, pi. iii. fig. 6. 



