236 



MR. R. LYDEKKER ON 



of monotremes and mammal-like reptiles, as is made clear by the 

 accompanying diagrammatic illustration (text-fig. 2), reproduced 

 from his fig. 19, p. 36. His researches, however, indicate that 

 when one of the two ventral elements in the shoulder-girdle 

 disappears, it is the posterior (and not, as previously supposed, the 

 anterior) bone which is lost. This degeneration — as a preliminary 



Text-figure 1. 



The right side of the shoulder-girdle of Dicynodon. 



sc, scapula ; a, acromion of scapula; c, coracoid ; mc, nietacoracoid ; f, coraco- 

 scapular foramen ; gl, glenoid cavity. 



to the ultimate disappearance — of the hinder element is exhibited 

 in his figure, which illustrates a type in which the degeneration 

 of that element has already become conspicuous, whereas in my 

 original figure (text-fig. 1) the hinder element is fully as 

 large as the front one. Consequently the element in birds and 



