224 PEOP. W. K. PAEKER ON THE 



time, the median from the lateral bony centres ; also in no other types than Eowls and 

 Hemipods are there ^«e definite bony tracts *. 



The old mistake of comparing the Jive bony centres of a Powl's sternum with the nine 

 dermal plates of a Tortoise, is scarcely worth mentioning in these latter days. But so 

 late and so excellent an observer as Miss Beatrice Lindsay has lost her way, now and 

 then, through not clearly seeing the difference between an endoskelctal cartilage and an 

 exoskeletal bony plate ; she has relied too much on dissections made before the tissues 

 had become fairly differentiated. We shall see how misleading this error is in what 

 is now to follow. 



In my paper on the " Shoulder-girdle and Sternum," I showed that the three simple 

 clavicles of a Lizard were the true homologues of the three foremost plates of the 

 plastron in the Chelonia; and that the three osseous centres that form the fur- 

 cula in all birds that have a median process to that compound bone belong to the 

 self-same category. But I also showed that the furcula, in most birds, was not merely 

 formed by fusion of these three bones — clavicles and interclavicle, but that in many 

 cases remnants or vestiges of the old Reptilian shoulder-plate — often bifenestrate in its 

 lower part — appear, become ossified, and then coalesce with the thin superficial clavicular 

 bones. For in this part of its structure, as in many others, the bird bears about the same 

 relation to a Reptile that the pupa of a Dragon-fly does to the culminating imago-form. 



In my early work, I laid too little stress on the meaning of the " clavicular process " 

 of the coracoid ; this part is aborted in the Common Fowl ; but, as I have shown in a 

 paper recently communicated to the Zoological Society, it is very large in the Hoatzin 

 [Opistliocomus). But besides that continuous remnant of the " precoracoid," there is a 

 separate segment of the same nature, but more external ; it is very large in the Cormorant 

 and its allies {op. c'd. plate 13. figs. 3-10, p.cr.) ; that is my precoracoid. Another 

 cartilage is developed in all the Passerines and in some of the Cuculines, namely, a 

 remnant of the acromion, a " meso-scapular segment " {op. cit. plate 15. figs. 12-15, 

 m.sc.s.). Miss Lindsay makes me call the down-bent head of the main coracoid the 

 " precoracoid ;" after speaking of this part in the Struthionidoe, she goes on to say : — 

 " The same uncertainty must exist with regard to the precoracoid of Diomedea (plate 44. 

 fig. 1); although there can be little doubt, from comparison with the early stages of 

 the various embryos figured, that we must agree with Sabatier in regarding this region 

 as the avian precoracoid, rather than the precoracoid of Parker, which Sabatier calls 

 an epiphysis " (o/;. cit. p. 705). Anything more confused than this it is impossible to 

 conceive ; the downturned bar of the coracoid of Diomedea and of other birds may be an 

 apophysis ; it has no ep)ip1iysis on it ; birds have very rarely more epiphyses than the right 

 and left cnemial centres. It certainly is not " the precoracoid of Parker." 



My critic does not directly say that all my figures of the interclavicle in my early work 

 (plates 13-17) are incorrect, but this is to be inferred from what she says on 



* The additional pair of bony centres figured hy me in Tiu-n/uv {op. cit. plate Ki. figs. 13, 14), and caUed " cora- 

 costea " (c. c), have no real existence ; after many years, re-examining this small sternum, I find that the lino thought 

 by mc to be a suture was only a fracture ! 



