THE DEVELOPMENT OF THE MARSUPIAL SHOULDER GIRDLE. 759 



there are only two well-marked borders. The endeavours which have been made to find 

 the homologues of the higher mammalian scapular borders in the lower forms have led to 

 very different conclusions. 



Most of the discussion concerning the morphology of the scapular borders has been 

 in connection with the scapula in the monotremes. In both Ornithorhynchus and 

 Echidna it is a flattened bone which bears a superficial resemblance to the eutherian 

 scapula ; but though at first sight the two borders look as though they might cor- 

 respond with the anterior and posterior borders of the typical mammalian scapula, 

 yet the muscular attachments are so dissimilar that the majority of anatomists have 

 been led to the opinion that neither of the seemingly similar borders are really 

 homologous. 



The whole subject has recently been very fully and most ably discussed by Wilson 

 and M'Kay (6). In 1893 M'Kay (7) published an exhaustive account of the morphology 

 of the muscles of the shoulder girdle of the monotremes, and in connection with the 

 same work, he and Wilson were led to examine the current views on the homology of 

 the borders and fossae of scapula of the monotremes. The border, which is usually 

 spoken of as the anterior, but which is really directed as much outwards as forwards, is 

 shown to agree as regards all its muscular attachments and relations with the " meso- 

 scapula " of the higher mammals, and also in its giving attachment to the clavicle. 

 This view has been held by Owen (8), Mivart (9), Parker (4), Flower (5), and Howes 

 (10), and there seems very good reason for believing it to be correct. Concerning the 

 homology of the posterior border, opinion has been much divided, and the facts are 

 doubtless less unequivocal. There is a well-marked ridge on the outer side of the 

 scapula of Echidna, and a corresponding though slightly less marked ridge on that 

 of Ornithorhynchus, situated above the glenoid cavity, which gives attachment to the 

 triceps muscle, and which divides the origins of the infra-spinatus and subscapularis 

 muscles. This ridge is held by Wilson and M'Kay to be the true homologue of the 

 posterior border of the eutherian scapula — a view which had previously been advocated 

 by Mivart and Flower. Owen, Parker, and most other anatomists, on the other hand, 

 have held that notwithstanding the differences in their muscular relations, the actual 

 posterior border in the monotremes is the morphological posterior border. These 

 differences of opinion are, however, slight in comparison with the differences which 

 have arisen in the endeavour to find in the monotremes the homologue of the 

 anterior or prsescapular border of the higher forms. On the inner side of the scapula 

 in Ornithorhynchus is a small ridge which represents the anterior border of the 

 origin of the subscapularis muscle, and in front of which arise the omo-hyoid and 

 the minute supra-spinatus. This ridge was regarded by Owen as the morphological 

 anterior border, and the same view is also held by Wilson and M'Kay. Flower, 

 whose opinion apparently has been mainly influenced by the condition in Echidna, 

 where the supra-spinatus muscle is of large size and occupies a very large part of the 

 inner surface of the scapula, and where the subscapularis arises almost entirely from the 



