MOEPHOLOGT OF OPISTHOCOMTJS CETSTATUS. 



73 



In the 2nd and 3rd stages (figs. 5-7) the main part has an ectosteal sheath, so that it 

 looks as if it were forming three segments. The ungual phalanx has now become a 

 mere cap of cartilage on the end of the main segment (fig. 5, dg?). In my paper on 

 the Fowl's Wing (pi. 64. figs. 12, 13, pi 65. figs. 1-3) I have shown that in Struthio 

 and Bhea there is a very small unguis on the third as well as on the first and second 

 digits. In the same paper (pis. 62-64.) I have shown, also, that in many cases there is 

 the rudiment of a fourth digit; this is cautiously lettered (dg.^) as an addendum to 

 the third metacarpal (mc.^'). It is a notable part in some old birds, for instance in 

 BhamjyJiastos toco and Bicholophus cristatus. Here, as in many other birds, I do not 

 find a fourth metacarpal, but there is a rudimentary phalangeal cartilage in all these 

 three stages (figs. 1, 5, 6, 7, dg^) ; it is most clearly seen in the 2nd stage (fig. 5). It 

 is a pyriform lobe of cartilage, with its narrow end forwards, and is adherent to the 

 ulnar edge of the abortive third finger ; it is about one third its size. Most of the 

 metamorphosis of a bird occurs in the encysted stage, whilst it is still in the egg, and 

 this takes place in a marvellously rapid manner. But during the first year, as I long ago 

 showed in my paper on the Fowl's Skull (Phil. Trans. 1869, pis. 81-87. pp. 755-897), 

 and afterwards to the end of life, in some degree slow obliterative change still 

 takes place through the gradual increase of bony substance. We shall look in vain in 

 the wing-bones of the adult Opisthocomus for much of what I have described in the 

 embryo. A comparison of this wing with that of a few other types will show that its 

 manus is unusually long. Notwithstanding the great distance of this bird from the 

 Macrochires, it has like them, but not to the same degree, a long distal segment to its 

 wing-skeleton ; in this respect it comes nearer to the Pteroclidse and Turnicidse than 

 to the Cracidge. This wing agrees with that of the Cracidse and Megapodidse (Peris- 

 teropodes), and also with the Pteroclidse, Turnicidee, and Tinamidse, in having the bridge 

 over the proximal part of the interosseous space of the manus (PL X. fig. 8, mc?') ; in 

 this, like them, it differs from the Alectoropodous Fowls, the Phasianidse, Tetraonidae, 

 &c. The length of the three main divisions of the wing-skeleton in six types is as 

 follows (all these are taken from adults, except Turnix rostrata, which was a chick of a 

 week or two old) : — 



Humerus. 



Opisthocomus cristatus . 



. 70 



Crax gloUcera . . . 



. 140 



Turnix rostrata . . . 



. 15-2 



Chauna chavaria . . . 



. 182 



Corythaix huffoni . . 



. 45 



Talegalla lathami . . 



. 84 



Cubitus. 



Manus. 



72-5 millim. 



69 millim 



150 



115 



12-4 „ 



18 



204 



155 



42 



37 „ 



86 



71 „ 



So that this type agrees with Crax, Chauna, and Talegalla in having the cubitus 

 larger than the humerus; its manus is but little less than its cubitus, differing in this 



