SKELETON OF COMMON PIGEON. 65 



The bones of the pelvis are separate inter se in Archaeopteryx. In all other 

 birds they fuse, at least in the acetabulum. The real length of the ilium is to be 

 measured from the outer ends of the sacral ribs to the acetabulum. The great 

 extension of the bone along the sacrum represents breadth, and is an exaggeration 

 of a Crocodilian and Deinosauran feature. The praeacetabular section, with few 

 exceptions, ossifies parosteally, and it is rarely shorter than the postacetabular 

 section, as in the Ostrich, Divers, and Ichthyornis. The length corresponds with 

 the iliac axis of Professor Huxley. As in all Sauropsida this axis makes an acute 

 angle forwards, not backwards as in Mammalia, with the sacral axis, i. e. a line 

 drawn antero-posteriorly through the centra of the sacral vertebrae. The dorsal 

 iliac area is little developed in some birds, e. g. Ostrich, Divers. 



The backward inclination of the pubes and ischia, and the loss of the ventral 

 symphysis is characteristic of birds among living Vertebrata. So far as the pubes 

 are concerned, there was no symphysis in many of the extinct Deinosauria, and 

 the same is true in some instances of the ischia also; and this is especially the 

 case in the two groups Stegosauria and Ornithopoda, in which the conformation 

 of the pelvis is Avian and the ischia with the main part of the pubes (post-pubes) 

 are inclined backwards. The Ostrich is the sole example of a bird with a pubic 

 symphysis, and in Rhea the ischia meet not ventrally, however, but dorsally imme- 

 diately under the backbone. The union between the distal ends of the pubis 

 and ischium of the same side, and of the latter with the ileum, does not always 

 occur, e. g. Cassowary, Emu, Apteryx among Ratitae ; the Tinnamou among 

 Carinatae. There is a well-developed pectineal process in front of the acetabulum 

 in the Ostrich. An examination of a young specimen shows that both ileum and 

 pubis enter into its formation. A similar process is found in some Carinatae; 

 but it appears to be formed entirely by the ileum in the Fowl. The chick, 

 however, as proved by Miss A. Johnson, has at an early period a large forward 

 extension of the pubis which gradually dwindles away. This process appears 

 to be the homologue of the prae-pubis (so-called) in the Stegosauria and Ornitho- 

 poda among Deinosauria, whilst the main portion of the bird's pubis is the 

 homologue of the post-pubis (so-called) in the same groups. There does not 

 appear to be much ground for supposing, (as has been done) that the prae- and 

 post-pubis represent separate bones. They are continuous one with the other in 

 Stegosaurus (cf. Marsh, American Journal of Science and Arts, xxi. p. 169, 

 PI. viii.), in Camptonotus (op. cit. xviii. p. 502), Laosaurus (op. cit. xvi. p. 415, 

 PI. x.), and in Iguanodon (Dollo, Bull. Mus. Roy. d'ru'stoire Naturelle de Belgique, 

 ii. 1883, PI. iii.). And the solitary instance of Allosaurus, which was supposed 

 by Marsh to have had a separate post-pubis (American Journal cited, xvii. p. 90, 

 PI. vii. 2), is now included by that author in a group of Carnivorous Deinosauria, 

 the Theropoda, in which the post-pubis is absent. See Marsh on Theropoda, 

 American Journal of Science, xxvii. 1884. Baur has recently suggested that the 

 pectineal process (in part) of Birds and of Deinosauria, or the prae-pubis in some 

 of the latter group, is the homologue of the Os acetabuli of Mammalia (see note, 

 p. 107, Z. A. ix. 1886). For the Os acetabuli, see Gegenbaur, Ausschluss des 

 Schambeins von der Pfanne, &c., M. J. ii. 1876; Krause, Centralblatt fur Me- 

 dicin. Wissenschaften, 1876, p. 817; Leche, Bronn's Klass. und Ordnungen des 

 Thierreichs, vi. pt. 5, Mammalia, p. 576; Id. Monthly Internat. Journal of Anat. 



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