ON THE TRUE CORACOID. 235 



18. The True Coracoid. 

 By the late R. Lydkkker, F.R.S., F.Z.S. 



[Received February 10, 1915 : Read March 23, 1915.] 



(Text-figures 1 & 2.) 



Morphology. 



In view of the long-standing existence of two diametrically 

 opposite interpretations of the homology of the ventral elements 

 or element * in the shoulder-girdle of vertebrates other than 

 fishes, it is high time that morphologists should decide which 

 they will adopt. It may be premised that the element (which- 

 ever it be) in the shoulder-girdle of the monotreme mammals and 

 Permo-Triassic mammal-like reptiles entitled to bear the desig- 

 nation coracoid must be the one corresponding to the coracoid 

 process of the human scapula, which is the type of that element. 

 By anatomists generally the posterior ventral bone in the mono- 

 treme shoulder-girdle has been regarded as representing the 

 true coracoid, and the anterior bone consequently considered 

 as a superadded element, under the designation of precoracoid or 

 epicoracoid ; the single ventral element in the shoulder-girdle of 

 birds and post-Triassic reptiles being identified with the one 

 termed coracoid in the monotremes. 



These identifications were disputed by myself in the Society's 

 Proceedings for 1893 (pp. 172-4), where, upon the evidence of a 

 distinct cora.coidal element in the shoulder-girclle of a sloth it 

 was held that the so-called epicoracoid of the monotremes and 

 mammal-like reptiles corresponds to the coracoid process of the 

 human scapula, and is thus the true coracoid. Consequently, the 

 bone in the aforesaid groups to which the latter name had been 

 applied must receive a new designation, and the name meta- 

 coracoid was suggested for use in this sense. These homologies 

 will be apparent from my original figure, of which a portion (text- 

 figure 1) is herewith reproduced. A further inference was that 

 when only a single ventral element is present in the shoulder- 

 girdle, as in birds and post-Triassic reptiles, this, on account of 

 having been identified with the posterior element in the mono- 

 treme shoulder-girdle, must also be a metacoracoid. 



Among the great majority of naturalists these identifications 

 have failed to gain acceptance. Recently, however, Prof. S. W. 

 Williston, of Chicago University, who has devoted special 

 attention to the osteology of the mammal-like reptiles, has accepted 

 in his ' Water Reptiles of the Past and Present/ f my inter- 

 pretation of the homology of the elements in the shoulder-girdle 



* When the singular is used, reference is to one side only of the body, 

 t Chicago, 1914. 



