630 



foetuses show that the higher mammalian coracoid process is the 

 homologue of the posterior element in the ancestor. The precoracoid 

 is never found in higher forms unless possibly as the coraco-cla- 

 vicular ligament. 



Other very early types show the humerus articulating mainly 

 with the scapula and the precoracoid and the true coracoid becoming 

 greatly reduced and then lost. The Cotylosaur Seymouria and the 

 later reptilian type Varanosaurus seem to give us the clue to the single 

 ventral element in most later reptiles and I think there can be no 

 reasonable doubt that Williston is right in holding that the single 

 element in lizards and birds is the homologue of the single element 

 in Yaranosaurus and that this is the precoracoid of those reptiles 

 which have two ventral elements. 



The diagrams given represent a series of types leading in the 

 one series to the single ventral element in the higher mammal which 

 is the coracoid, and in the other series to the single element of the 

 lizard which is the precoracoid. 



The following table will illustrate the changes in view of the 

 coracoid homologies. 



The reason why Paekee, Howes, and most other morphologists 

 have hesitated in believing the anterior element of the Monotreme 

 and Anomodont shoulder girdle to be the precoracoid is because they 

 beüeve that the precoracoid of the amphibian becomes a cartilaginous 

 basis for the clavicle. Gegenbaue in 1864 believed he had shown 

 that the human clavicle is developed in cartilage; and this idea has 

 passed into nearly every text book written since. Unfortunately, as 

 I have shown, Gegenbaue's observation is incorrect. In no animal has 

 cartilage ever been detected by any one in connection with the clavicle 

 till after the bone is Avell ossified. Even in the mole where cartilage 

 later on developes to such an extent that Paeker regarded the clavicle 

 as partly coracoid there is no trace of cartilage till the clavicle is of 

 large size. In Marsupials and Reptiles there is no trace of cartilage 



