30 ARBOREAL MAN 



being likened by all comparative anatomists to that in 

 the typical pre-mammalian Vertebrates. In the rest of 

 the mammalian orders the appearance of the clavicle, 

 if we take the systematic posi- 

 tion of the animal as our guide, 

 can only be described as hap- 

 hazard. Among the Metatheria 

 (Marsupials), only Perameles (the 

 Bandicoot) fails to possess a 

 clavicle. Among Eutheria (higher 

 Mammals), a complete clavicle 

 is present in all Insectivora, 

 except the aberrent aquatic 

 Potamogale ; it is present in some 

 Edentata, in all Cheiroptera, and 

 all Primates. It is entirely 

 absent in Cetacea and Sirenia 

 and Ungulata (see Fig. 10); in 

 most Carnivora it appears only 

 as a rudiment, though in some 

 members of this order it attains 

 a fair degree of development; 

 in the Rodentia it is sometimes 

 well developed and sometimes 

 entirely absent. The only under- 

 lying principle which seems to 

 FIG. 10. THE SHOULDER explain the rat her random de- 



GlRDLE OF AN UNGU- f - ,, . , j-.cc 



LATE, TO SHOW THE velopment of this bone in differ- 



ABSENCE OF THE CLAV- ent Mammals appears to be 



ICLE. found in the functional demands 



The fore-limb has no strut made upon the movements of 



to keep it poised at the the fore-limb. So long as no 



more demand is made than the 



simple backward and forward movement at the shoulder- 

 joint, such as is seen in the walking and trotting of pure 

 quadrupeds, this strut is either not developed, or attains 

 no greater perfection than that of a mere isolated inter - 



