CHAPTER Y. 



THE PELVIC ARCH OF HESPERORNIS. 

 (Plates X-XI and XX.) 



The pelvic arch of Hesperornis exhibits many features of interest, and 

 characters more distinctly reptilian than that of any recent bird. In its 

 g-eneral form, the pelvis of Hesperornis regalis resembles that of Podiceps. 

 It is very long and narrow, as in that genus, and in other diving birds. 

 The acetabulum diifers from that in all known birds, in being closed 

 internally by bone, except a foramen that perforates the inner wall, as in 

 the Crocodiles. The ilium, ischium, and pubis, moreover, have their 

 posterior extremities free and distinct. This reptilian character is seen, 

 likewise, in the Emeu, as well as in Tinamus, which in other respects also 

 shows affinities with the Ostriches. 



The Ilium. (Plates X and XI.) 



The ilium is extremely long and narrow. Its superior outline is 

 gently arcuate, and this border joins the dorsal margin of the other ilium 

 on the median line, thus forming a roof-shaped covering over nearly all 

 the sacrum. The pre-acetabular portion of the ilium is only about one- 

 fourth of its entire extent. From the acetabulum forward, the ilia diverge, 

 showing at first, between their upper margins, the neural spines of the 

 vertebra? below, and then, in front, the zygapophyses of the first of the 

 sacral series. The anterior extremity of the ilium is thin, and rounded in 

 outline, the ventral margin extending nearly to the bottom of the first 

 sacral vertebra. 



