68 PROF. W. K. PAEKEE ON THE 



coracoidal flap lies over the clavicle ; the often massive segment, which is manifestly part 

 of the original bar of an archaic type, lies below^ or behind the clavicle (see in Pliala- 

 crocorax and Sula, op. cit. pi. xiii. figs. 3-10, ^^.cr.), and forms, after separate ossification 

 and fusion with the clavicle, the flat-topped shoulder of the furcular ramus in those 

 types. This addition to the clavicle articulates with the head of the coracoid, under- 

 propping it, and this is the part which in birds I have constantly called the pre- 

 coracoid ^. 



But there is in some cases another nucleus of cartilage at the top of the clavicle ; 

 this I have called the " meso-scapular segment" [op. cit. pi. xv. figs. 12-15, m.sc.s.), 

 for it is manifestly a segment from the meso-scapular or acromial region of the shoulder- 

 plate. It gives rise to the enlargement of the upper and posterior lobe of the wide- 

 topped furcular ramus of all true Passerine birds and of some of the Coccygomorphse, 

 for instance Rhamphastos, Picus, Alcedo, &c. I mention these specializations in the 

 most specialized kinds of birds to show how low is the ornithic level of Opisthocomus ; 

 its furcula will reveal this in a still clearer manner. The furcula of Opisthocomus is 

 composed of three parosteal bones, two clavicles and an interclavicle ; and notwith- 

 standing the figures and descriptions given by me long ago, in my large memoir, of a 

 distinct interclavicle in Birds, answering to the long bone of Lizards, Prof. Huxley {op. 

 cit. p. 307) has been careful not to use the term, but calls the stem of the Y-shaped 

 merrythought the " hypocleidium." A younger biological sceptic, Miss Lindsay, also 

 doubts my interpretation. She says, speaking of the chick of the 8th day: — "The 

 ribs at any rate have established their generic characters at this date, which renders 

 it probable that the broadening ossification of the median region of the clavicles, 

 described by Gotte as established during the 8th day, is an outgrowth of the Avian 

 furcula rather than a pre- Avian interclavicle — a view which is expressed by giving to it 

 (as has been done throughout this paper) the name of median furcular apophysis rather 

 than oi interclavicle " {op. cit. p. 702). 



Opisthocomus has, of all birds, the most perfect Y-shaped furcula (PL IX. figs. 1-3, 

 cl.i.cl.) ; its forks and stem are of equal length. 



The forks or rami are somewhat sigmoid, are narrow, and bent outwards at the top ; 

 they suddenly flatten out and then gently become narrower, the lower part being only 

 half as wide as the upper. The two clavicles not only close in the semi-oval space, 

 they also run down the stem for half its length, lying close together above, and then 

 becoming slightly separate. Into this space there fits a fine needle of bone, the 

 interclavicle ; it is two-thirds the length of the stem ; thus the hypocleidium is composed 

 of three elements. This interclavicle binds the furcula to the lower face of the sternum, 

 as in the Lizard [op. cit. pis. viii.-xi.) ; it can be seen through the transparent cartilage 

 in this first stage. In Lizards it often gets in between the right and left halves of the 



' See op. cit. p. 150. There I remark that " the line of segmentation between the coracoid and the great 

 precoracoid segment has become a large oval, gliding, synovial joint." 



