MOEPHOLOGY OF OPISTHOCOMUS CRISTATUS. 09 



sternum, and appears on the upper surface (see in Lcemanctus, op. cit. pi. ix. fig. 9, i.cl.). 

 A delicate ligament, half the length of the bony style, connects it with the arrested 

 sternal keel. In the adult bird these three elements are fused into the single furcula^ 

 and this bone becomes ankylosed above with the coracoids, and below with the sternum, 

 both rostrum and body; the scapula only remains distinct. 



This arboreal bird has evidently only developed its organs of flight for flitting ; the 

 great amount of ankylosis in its shoulder-girdle suggests this. In my notes on these 

 parts in the Hunterian specimen (old male) I have remarked that " the ankylosis has 

 not obliterated the lines of junction of the large clavicular process of the coracoid with 

 the clavicle, nor of the head of the latter with the acromial process of the scapula, for 

 that bone remains free whilst the others are all melted together — clavicles, interclavicles, 

 coracoids, and sternum." 



VI. The Wings of Opisthocomus cristatus. 



In the 1st stage (PL X, figs. 1-3) the wing is as much developed as in my 5th stage 

 of the Common Fowl (Phil. Trans. 1888, B, pi. 62. fig. 7}\ 



It is necessary for me to refer to those figures, for in them is shown what I find to 

 be constant in all the birds I have studied at that stage, namely that there are at first 

 only two proximal carpals. In the paper referred to I have called these the " inter- 

 medio-radiale " and the " centralo-ulnare ; " in the adult bird the ordinary nomenclature 

 for these carpals is simply " radiale " and " ulnare." It is necessary to remember that 

 in the strangely adaptive abortion of many elements of the bird's fore limb, some parts 

 are primarily, and some secondarily developed, the latter by segmentation of a 

 cartilaginous mass; and this is often very temporary, the parts becoming fused 

 together again, even, in some cases, before the time of hatching. 



The wing of Opisthocomus, although very much like that of a Fowl in its adult 

 state, yet differs much from it in its development. The long cartilages in the first 

 stage have already a considerable ectosteal sheath (Pis. VII., X. figs. 1-3). As in the 

 true Fowls, the humerus and cubitus differ but little in length ; they are quite normal, 

 and need not be described in this stage. The manus, however, shows many parts that 

 are lost and leave no sign in the adult ; all these unlooked-for segments are, never- 

 theless, quite normal parts of a typical cheiropterygium. The two nuclei of cartilage 

 seen in my early stages of the chick attached to the radius and ulna are here seen as 

 Jive more or less distinct tracts (PI. X. figs. 1, 2). The radiale {re.) is now a pedate 

 wedge of cartilage lying on the fore margin of the wrist, and most to the flexor or 

 inner side. The intermedium (^.) is a four-sided short wedge, somewhat lesser than the 



■ 1 In figs. 1 & 2, 1st and 2nd stages in that paper, the distal carpal {cl.c.'^) is drawn too near the first meta- 

 carpal ())i.c.'). In fig. 3 the true position is given, namelj' inside the head of the great metacarpal. The first 

 distal carpal is always developed in birds as a free nucleus of cartilage in that position ; it is a feeble element, 

 displaced to the flexor side of the limb. 



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