BY J. T. WILSON AND W. J. S. McKAY. 385 



reaches as far as the ridge and is not confined to the actual 

 posterior margin of the blade. Gal ton's description of the 

 condition in this animal does not differ from Humphry's in any 

 essential feature (14). 



Many rodents also exhibit a similar though less notable post- 

 scapular spinous development, as may be well seen in the genus 

 Arctomys [(6) pi. lxxi. fig. 11, and (8) pi. xxiv. fig. 14]. 



If Giebel's interpretation of the postscapular spine in Edentata 

 be correct, we can then bring the monotrerae condition in this 

 respect well into line with the latter, as may be seen from a 

 comparison of the series of figures 5-8."^ 



If the views we have expressed are correct, then there can be 

 Jittle difficulty in homologising the two forms of monotreme 

 scapula. In both forms the actual anterior border is m( so- 

 scapular. In both the prescapular part of the bone is suppressed, 

 though in Platypus its site is still indicated by a ridge on the 

 inner surface. In both scapulae the actual posterior border is 

 really secondary, being an exaggerated subscapular ridge, the 

 subscapularis muscle taking origin both from the ridge itself and 

 the bone on both sides of it. Finally, and as a consequence of 

 the last character, the postscapular border is displaced outwards 

 and exists as a mere ' tricipital ' ridge upon the outer surface of 

 the flat bladebone. 



Figs. 5-8 diagrammatically illustrate the points just summarised. 



* We have already casually referred to the extraordinary view propounded 

 by Brlihl (7) that the postscapular ridge in question (tricipital) is " die bei 

 Echidna deutliche Spina." In Omithorhynchus he less confidently suggests 

 a like view of the corresponding ridge. This theory not only ignores Owen's 

 view (accepted by Flower) of the true spinous (mesoscapular) character of 

 the actual anterior margin, but reveals either an entire ignorance of the 

 muscular attachments of the scapula or a very extraordinary method in 

 their interpretation. Thus in Echidna he suggests that the surface in 

 front of this (tricipital or postscapular) ridge is a prespinous fossa, possibly 

 homologous to the supraspinous fossa of higher mammals. But it is this 

 very area which is occupied by the origin of the infraspinatus muscle ; 

 while tiie area behind the same ridge, regarded by Briihl as postspinous 

 and possibly homologous to the infraspinous region, is occupied by part of 

 the subscapularis muscle. 



