762 DR R. BROOM ON 



M'Kay, a new development, is more difficult of solution. If the question is to be 

 decided by muscular attachments, no doubt the evidence would favour the homology of 

 the tj-pical mammalian posterior border with the little ridge on the outer side of the 

 scapula of the monotremes. If, however, we admit the possibility of the plane of the 

 scapula becoming partly rotated, and this we unquestionably find in some of the 

 higher mammals {e.g., Ch rysochloris, where the praescapular border is carried inwards), it 

 will be seen that such a rotation would compel certain muscles to take up new attach- 

 ments. It can hardly be denied that the anterior border of the scapula in the 

 monotreme is the homologue of the anterior border in the Theromorph, and yet while 

 this border in the ancestral forms is directed forwards, in the monotremes it looks more 

 outwards than forwards. And if the anterior border has thus been rotated outwards it 

 seems but reasonable to suppose that the posterior border has been correspondingly 

 rotated inwards. In both monotremes, the change has probably been brought about by 

 the digging habits of the genera. If we now suppose this change to have taken place, it 

 will be seen that the triceps muscle must of necessity take up a point of attachment on 

 the outer side of the scapula, as, were it to continue attached to the posterior border, the 

 action of the muscle would be hampered. The inward rotation of the posterior border 

 also of necessity in Echidna brings almost the whole of the subscapularis muscle to the 

 outer side, as it would be practically impossible for the muscle to work round the 

 posterior border in its present attitude. In Ornithorhynchus, owing to the upper and 

 posterior part of the scapula being almost sickle-shaped, a large portion of the sub- 

 scapularis muscle is enabled to retain its original attachment and to work clear of the 

 lower part of the posterior border. The muscles not connected with the arm would be 

 less affected by the altered attitude of the blade, and we find that the serratus magnus 

 and rhomboideus practically retain their original attachments. 



It will thus be seen that there is good reason for believing both the monotreme and 

 the marsupial types of scapula to have been derived from the primitive Theromorphine 

 type ; the monotreme type being formed by the coalescence of the cleithrum to the 

 morphological anterior border of the scapula, complicated by a slight rotation of the 

 plane of the blade on its long axis ; the marsupial type, on the other hand, being 

 formed by the cleithrum coalescing to the scapula on its outer surface, leaving the 

 anterior border of the scapula free, and thus giving rise to a new fossa — that between 

 the cleithrum or spine and the anterior scapular border. In the Cetacea we have 

 apparently a scapula in which, though the acromion is well developed, there is no true 

 spine, the infraspinous fossa extending to the anterior border of the blade, and the 

 supra-spinatus muscle occupying a little hollow on the inner side of the acromion. 



It is, of course, not improbable that the cleithrum element may be completely lost in 

 the monotremes. If the study of the early development prove this to be so, the actual 

 anterior border must be the morphological anterior border. 



