764 DJR E. BROOM ON 



and it is this same element which serves to fix the shoulder girdle by articulating 

 with the sternum immediately in front of the first rib. In the fcetal marsupial the 

 relations and functions of the cartilaginous element are precisely similar, and I am 

 aware of no fact or argument which seems to me to be opposed to this element being 

 the true homologue of the posterior of the two coracoidal elements in the monotreme. 



If the cartilaginous ventral element in the fcetal marsupial be not the homologue of 

 the posterior element in the monotreme, it must either represent a fusion of the coracoid 

 and precoracoid, or be the homologue of the precoracoid. As no scientist, so far as I am 

 aware, holds the view that the mammalian coracoid process represents two elements, 

 it will be unnecessary to discuss this alternative. Though Howes holds that both 

 coracoidal elements are present in the typical eutherian shoulder girdle, he admits that 

 the ossification which forms the coracoid process is a single element, and there is no 

 doubt that the cartilaginous process in the fcetal marsupial represents this one element. 

 The remaining alternative, that the process represents the precoracoid (' epicoracoid '), 

 is the view held by both Howes and Lydekker. Howes, as we have seen, was led to 

 this view by the discovery of a second centre of ossification which he took for the true 

 coracoid. If we assume the well-developed cartilaginous ventral element in the foetal 

 marsupial to be the precoracoid, any rudiment of the true coracoid which might be 

 present we would expect to find on the posterior side of the well-developed element, 

 and, of course, on the inner side of the glenoid cavity ; but the little ossification which 

 Howes believes to represent the coracoid lies on the outer side of the glenoid cavity. 

 If Howes' view be correct, the following most improbable changes must have taken 

 place in order to have the mammalian shoulder girdle derived from that of the 

 Theromorph : — 1st, the great reduction of the element which is most useful in uniting 

 the scapula with the sternum ; 2nd, its exact place and uses being taken up by the 

 precoracoid ; and 3rd, the rudimentary true coracoid being shifted from the inner side 

 of the glenoid cavity to the outer. As I can see no reason for believing that any of 

 these changes has taken place, and many reasons for believing that such remarkable 

 changes have not taken place, I feel compelled to accept the view of the majority of 

 earlier writers, that the coracoid process of the marsupials and all higher mammals is 

 the true homologue of the posterior of the two elements in the monotremes and 

 theromorphs. 



In Lydekker's comparison of the shoulder girdle of the sloth with that of Dicy- 

 nodou, he concludes that the coracoid of the sloth is the homologue of the precoracoid 

 (' epicoracoid ') in Dicynodon, because of " each articulating with the lower border of the 

 front of the scapula, from which they are partially separated by a foramen, and each 

 entering into the formation of the glenoid cavity." But if the shoulder girdle of 

 Plesiosaurus or of CryjDtoclidus be compared with that of Dicynodon it will be seen that 

 the very same argument would serve to prove that the element which articulates with 

 the scapula in these Sauropterygians was a precoracoid — a view which could hardly 

 be accepted. 



