78 FRANK BLAIR HANSON 



5. Monotremes 



The monotremes (fig. 4) have long been held to have a sternum 

 closely reptilian in character. In addition to the coracoids, 

 which are firmly attached to the sternum, there is also an an- 

 terior paired element, usually called the epicoracoid. In these 

 there is an overlapping in the midline much as in certain amphib- 

 ians, and in a recent paper Watson ('17) holds that the epicora- 

 coids of the monotremes are nothing other than the precoracoids 

 of the lower forms. That typically reptilian structure, the inter- 

 clavicle, is also present. As seen in the figure, there is notice- 

 able a compactness of the elements of the shoulder-girdle and 

 sternum, as if these might have been in the early embryo or in 

 the ancient progenitor a single shield-like plate, such as occurs 

 in Pipa (fig. 20) or in many of the reptiles. 



6. Marsupials 



In 1897 Broom discovered in the marsupial Trichosurus vul- 

 pecula, measuring 17 mm., a well-developed coracoid, which was 

 at birth "structurally continuous with the sternum. " Figure 35 

 is an anterior view of the entire scapular arch which in general 

 outline is strikingly like that of the shark. Its ventral middle 

 portion is a part of the sternum, yet the parts are, as Broom says, 

 not jointed, but constitute a single bar of cartilage. 



In a smaller specimen (fig. 36) of 8.5 mm. of the same animal 

 both cartilaginous and mesenchymatous elements are present in 

 the coracoid; the cartilaginous part being that nearest the glenoid 

 cavity, and the mesenchymatous spreading out in a fan-shape 

 portion, which is continued without any interruption into the 

 mesenchymatous sternum. It is apparent that this marsupial 

 has a complete scapular arch crossing the middle line ; and that, 

 from the history of the later stages, the sternum as well as the 

 girdles are known to be its derivatives. Furthermore, just in 

 front of the coracoid and posterior to the clavicle, there was "a 

 thin, feebly developed continuous sheet of mesenchymatous 

 cells;" lying therefore in the exact position of that anterior cora- 

 coidal element of the monotremes generally called the epicoracoid, 



