BY J. T. WILSON AND W. J. S. McKAY. 379 



be admitted the prescapular ridge is not represented by any- 

 recognisable mark. According to Owen (loc. cit.) it is "nearly 

 obsolete " in Echidna. We hope to show, however, that, notwith- 

 standing the absence of a distinct prescapular ridge, the condition 

 in Echidna (as regards muscular attachments, &c.) does so essen- 

 tially resemble that in Ornithorhynchus that it is an exceedingly 

 simple matter to homologise the parts in the two genera. And 

 if we interpret the ridge on the inner surface of the scapula of 

 Ornithorhynchus (after Owen) as the true morphological anterior 

 border (prescapular), it becomes comparatively easy to exhibit the 

 real correspondence of the borders and surfaces of the scapula in 

 monotremes to those in the typical mammalian scapula. 



The identification of an area of the ' outer ' surface of the 

 scapula, by reason of its giving origin to the infraspinatus muscle, 

 as ' postscapular fossa,' has already been alluded to. In Ornitho- 

 rhynchus this postscapular 'fossa' extends from the actual anterior 

 (true mesoscapular) margin of the bone as far back as the ridge 

 already referred to as giving attachment to the scapular triceps. 

 The like is true also of the scapula in Echidna, but here the 

 tricipital ridge does not lie so far back as it does in the Platypus, 

 but is separated by a considerable interval from the actual hinder 

 border of the bone. In fact the ' outer ' surface of the scapula in 

 Echidna is nearly bisected into preaxial and postaxial halves by 

 the tricipital ridge, which is more strongly marked than in 

 Ornithorhynchus. 



It may now be asked how the mode of attachment of the supra- 

 si)inatus muscle agrees with the view so far expressed respecting 

 the homologies of the parts of the bone. Naturally we should 

 expect to find it associated with that part of the bone which 

 represents the region of the prescapular fossa ; and this, it will 

 be remembered, we regard as that area of the inner surface of the 

 scapula in Ornithorhynchus which lies between the actual anterior 

 (mesoscapular) border and Owen's 'true anterior costa,' ^.e., the 

 ridge formerly described. But the supraspinatus in Ornitho- 

 rhynchus is a relatively very minute muscle, and it does not by 

 any means occupy the whole of the above area. Its origin is 

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