434 BuUctin American Museum, of Natural History, [^^ol. XXVII, 



part of the humeral trochlea in primitive mammals on its posterior face 

 receives the internal process of the ulna. In many forms this internal 

 process gradually extends forward on to the anterior face of the trochlea, 

 and in Man causes the radial capitellum to be shifted to the extreme outer 

 side of the humerus. This conception is directly in contradiction to that of 

 Flower, who says (/. c, p. 269) that in the "primitive or unmodified condi- 

 tion" the "radius articulates above with the i)reaxial (external) side of the 

 humerus, the ulna with the postaxial (internal) side of the humerus." To 

 him the primitive relations are "best illustrated in the fore-limb of the 

 Cetacea" where the radial or preaxial border is "external," the ulnar 

 postaxial border " internal." But the " Cetoid nature of the promammalia" 

 (Albrecht) is a thoroughly discredited theory (p. 414), and as stated above, 

 the evidence from many lower vertebrates shows that the primitive position 

 of the ulna with respect to the humeral trochlea is wholly behind, and not 

 side by side with, the radius. In brief, the preaxial border in the mammalian 

 fore limb includes the following elements which on the theory that the 

 humerus originally pointed backward and outward (see above) were all 

 primitively more or less antero-internal with respect to the axes of the body: 



Digit I. 



Scaphoid. 



"Front" face of radius (contrast Flower, fig. 133, p. 364.) 



" " humeral trochlea (contrast Flower, fig. 133, p. 364). 

 " humerus, including the deltoid crest (contrast Flower, 

 fig. 133, p. 364). 



Thus the entocondyle and ectocondyle were neither preaxial nor post- 

 axial (contrast Flower's diagram, /. c., p. 364) but in a i)lane nearly at right 

 angles to the plane of the radius and ulna. 



The "postaxial" (postero-external) border included the following ele- 

 ments : 



Digit V. 



Cuneiform. 



Ulna. 



Anconeal fossa. 



"Back" (dorsal) face of humerus (contrast Flower, p. 365, fig. 134 G.). 



The "preaxial" border of the scapula was antero-external in position, 

 and was homologous with the spine and acromion of the scapula in higher 

 mammals. The "postaxial" border of the scapula was posterior or postero- 

 internal in position. 



The evolution of the fore limb from the Prototherian to the Metatherian 

 stage may possibly have been conditioned by the assumption of arboreal 

 habits {cf. p. 226). It involved chiefly the following factors: (1) the back- 



