MORPHOLOGY OF OPISTHOCOMUS CEISTATUS. 63 



aborted, and of the eight uro-sacrals the first two have riblets and diapophyses; 

 Cd. 6 + 3: Total 45. 



This remark may be made, namely that the same bird diff'ers very much in youth 

 and a^e ; small riblets are apt to lose their distinctness or to become starved ; while 

 the diapophyses themselves often shrink, so that a strong buttress may become a small 

 prickle, which often quite disappears on one side. The length of the pre-iliac region 

 of the sacrum in Cliauna is a Cygnine character ; indeed it has one more vertebra in 

 the dorso-lumbo-sacral series than the Swan [Cygnus olor). Opisthocomus agrees with 

 that Neotropical generalized Chenomorph — Chaima — in the length of its neck, a length 

 which attains its highest condition in the Swan, which has 25 cervical vertebrae ; in 

 the other regions Opisthocomus comes near to the Gallinacese generally. The reniform 

 occipital condyle in this bird fits into a notched atlantal cup as in the Fowls. The 

 neural spines begin on the second vertebra, die out on the eighth, reappear on the 

 thirteenth and fourteenth, and become complete on the seventeenth. 



The axis and the two next have a small, thick inferior spine, which is nearly obsolete 

 on the fifth. There is a small inferior spine on the fifteenth and sixteenth, and there 

 is a free median spine with a pair of lateral ridges on the last three cervicals. The 

 dorsals are peculiar ; they are cylindroidal in their articulation, like the cervicals, and 

 are flat-bottomed. There are no carotid bridges below in the cervical region. The 

 first two or rib-bearing sacrals (dorso-sacrals) are somewhat bicarinate ; the rest have 

 merely the usual convex form. Keturning to the cervical region there is a structure 

 which is very remarkable, and yet not rare in the Carinatee. The seventh to the 

 twelfth inclusive have an oblique bony lamina outside the canal for the vertebral 

 artery, a kind of flying buttress. In Crax globicera this is seen on the sixth to the 

 ninth inclusive ; this structure has its greatest development in certain of the Coccygo- 

 morpha;. The cervical riblets are small, but have a large base between the diapophyses 

 and the parapophyses ; they only form small additions to the former processes in the 

 sixteenth and seventeenth ; altogether the neck is very gallinaceous except as to its 

 length. The sacral region is intensely ossified ; the spine never quite dies out even in 

 the true sacrals. Gradually the intervertebral spaces appear behind ; but the main 

 part of the pelvis above is largely plastered over with periosteal bone. The caudal 

 vertebrsB are not pneumatic, the rest of the spine is ; the transverse and upper processes 

 of the caudals are thick and of moderate size ; the centra have the usual development. 

 The intercentra are developed all along : that of the first is fused with the last sacral 

 centrum ; the second is a small grain of bone ; the rest form thick inferior spines to 

 the vertebrae; this is seen also on the uropygial bone, which is formed of four segments 

 in the embryo (PI. IX. fig. 5); it is thick below as in Crax ; it is somewhat hooked 

 downwards at the tip. The ribs in the old bird are strong bars; those on the 

 eio-hteenth cervical are 24 millim. long, and have no uncinate processes; those on the 



