7G0 



DR R. BROOM ON 



outer side of the bone, regards the actual posterior border as the true homologue of the 

 anterior border in the higher mammals ; while Parker's opinion is that in the mono- 

 tremes " there is no praescapula." 



The leading views that have been held as to the homologues in the monotremes 

 of the borders of the eutherian scapula are shown in the following table : — 



Owen (1847) 



Prrescapular Border. 



Mesoscapular Border. 



Postscapular Bonier. 



Ridge on inner side of scapula 

 in Ornithorhynchus 



Anterior border 



Posterior border 



Parker (1868) 



Absent 



Anterior border 



Posterior border 



Flower (1870) 



Posterior border 



Anterior border 



Tricipital ridge 



Bruhl(11)(1875) ... 



Anterior border 



Tricipital ridge 



Posterior border 



Wilson&M'Kay(1893) 



Ridge on inner side of scapula 

 in Ornithorhynchus 



Anterior border 



Tricipital ridge 



It will be seen that the differences of view have arisen owing to the differences 

 of opinion as to what value is to be placed on the positions of the muscular attachments. 

 In considering the scapula of the monotremes in reference to the muscular attachments, 

 it is important to keep in mind that in neither Echidna nor in Ornithorhynchus does 

 the scapula lie, as in the higher mammals, by the side of the chest, and in an antero- 

 posterior plane, but that in both the plane of the scapula is oblique, the posterior 

 border looking about as much inwards as backwards, and the anterior border being 

 directed mainly outwards. Furthermore, the long axis of the scapula, instead of, as 

 in the higher forms, pointing upwards and backwards, points upwards and forwards 

 — the scapulas embracing the neck well in front of the chest. When this different 

 attitude of the scapula is considered, it seems to me a more natural explanation of the 

 peculiarities, that the muscles had to alter their attachments, than that the bone had 

 become developed in new axes. 



It may be regarded as moderately certain that the scapula of the higher mammals 

 is not derived from that of the monotremes, but it is not difficult to derive the 

 monotreme type from what may be regarded as the ancestral mammalian type. Let 

 us consider the structure of the scapula in this primitive type. We are familiar with 

 the structure of the shoulder girdle in Pareiasaurus and Dicynodon, and though 

 probably neither of these genera is an ancestral type, their scapulas are so similar that 

 it seems probable that in the Theromorphine types, from which the mammals are 

 derived, the scapula differed but little from that of the forms already known. In 

 these types the scapula is a flattened, elongated blade, with but two distinct borders, 

 an anterior and a posterior. There seems good reason to believe that the long axis of 

 the scapula was directed upwards and slightly forwards. Seeley, however, it must be 

 admitted, opposes this view, and in his (12) restorations of Rhopalodon and Deutcro- 



