16G Report 8. A. A. Advancement of Science. 



the mammalian sternum, it seems to me advisable provisionally to 

 retain the old name. 



In connection with the shouldei-giidle of the 8tegocephalia there 

 is still some confusion. In the large majority of known genera there 

 are seven ossified elements — a plastron formed by a median shield and 

 two lateral plates, two long slender elements and a pair of semicircular 

 bones. A considerable number of different names have been employed 

 foi- the three median plates, and a list of a number appears in a recent 

 paper by Thevenin. The old view expressed in the British Association 

 report on Labyrinthodonts in 1873, that the three elements are the 

 interclavicle and the two clavicles, has been so full}- confirmed by recent 

 investigations that there seems no longer any room for doubt on the 

 matter. With regard to the other two pairs of bones the difference of 

 opinion is great. By some the semicircular one is supposed to be the 

 coracoid, and the slender one the scapula ; by others the interpretations 

 are reversed. All recent paUeontological work has, however, rendered 

 it more and more evident that the slender element is a membrane bone, 

 and Gegenbaur has given to it the name of cleifhrum. It may be the 

 same as the supraclavicle of the bony fishes, but it seems better to use 

 Gegenbaur's name to avoid any possible confusion. In the higher 

 Labyrinthodonts we see the cleithrum forming a protection for the 

 front of the scapula, and we can trace the bone through a number of 

 the higher groups of reptiles. It appears to occur in the Cotylosauria 

 and in the Pelycosauria, and a well-developed cleithrum is present in 

 Pareiaitanrii^. Among the Anonuxlonts a small but distinct cleithrun> 

 is found in Divynodon. 



The semicircular element is evidently a bone formed from car- 

 tilage, and is the only ossification of what was doubtless a large 

 cartilaginous shoulder-girdle. Either it is the scapula or the coracoid. 

 If we look at the shoulder-girdle of those Labyrinthodonts where the 

 girdle is fully ossified we find the scapula is laige and the coracoid 

 small, and if we examine those specimens of the earlier forms where 

 the parts are little disturbed we see that the head of the humerus lies 

 inside and behind tlie semicircular ossification. We are therefore 

 probably safe in concluding that the bone is scapula and that the 

 coracoid and precoracoid remained cartilaginous. This view is further 

 confirmed by a comparison with the shoulder-girdle of the lower 

 Un»dela, such as I'ruteuf, where we find the scapula alone ossified, and 

 with that of Fa/nohaiferia, where we find a very similar semicircular 

 bone which is certainly scapula. 



