THE CLAVICLE 29 



It may in some fishes become complicated by other 

 struts not primitively parts of the shoulder girdle, or it 

 may strut the shoulder girdle, not to the sternum, but 

 to the skull, or to some outlier of it. A clavicular strut 

 derived from dermal bone is present also in the majority 



FIG. 9. THE HUMAN SHOULDER GIRDLE, TO snow THE STRUT 

 ACTION OF THE CLAVICLE. 



of the Amphibians and Reptiles. Within the limits of 

 these groups great range of variation in development is 

 seen; the dermal struts may attain great complexity, or 

 they may be altogether absent, and, on the whole, a 

 functional rather than a systematic cause underlies the 

 degree and condition of their presence. The dermal 

 strut occurs again in the lowest Mammals, and here in 

 perfect reptilian complexity of structure, the condition 

 present in the Prototheria (Duck-billed Platypus, etc.) 



