Cope.] ^^4: [Oct. 7, 



that of the diplarthrous Ungulata (Fig. E). Here the head of the radius, 

 while retaining its normal position on the inner side of the forearm, is 

 extended to the external side of the ulna and even beyond it, adapting it- 

 self to the entire width of the humeral condyles. The same structure is 

 found in the specialized forms of both series of Diplarthra, the Perisso- 

 dactyla and Artiodactyla. This expansion of the head of the radius ap- 

 pears to be in direct relation to the duration through long geologic ages 

 of tiie impacts which have affected the limbs of these, the swiftest of the 

 Mammalia. That the head of the radius should be spread so as to fit the 

 entire surface of the humerus as an effect of continued impact, seems to 

 be a mechanical necessity. But in addition to this we find a tongue-and- 

 groove adaptation In which the crest (which I have called the trochlear 

 crest), articulates with a groove in the head of the radius. The internal 

 articulation of the humerus with the radius has the usual form, convex 

 and concave dislad. Tlie trochlear crest marks the external border of 

 the olecranar groove of the humerus. But the external part of the 

 humeral condyles is converted into a roller which is set off from the 

 trochlear crest, by the abrupt contraction of its diameter ; while the cor- 

 responding part of the head of the radius projects to fit it exactly. 



A probable explanation of the form of this roller may be derived from 

 a consideration of the almost identical structure of the metapodio-phalan- 

 geal articulation in the Artiodactyla. The internal and external sides of 

 the distal metapodial condyles are not similar ; a character very distinct 

 in the Artiodactyla (Fig. E). I'his is simply due to the unequal pres- 

 sure exerted on the two extremities of the condyle by the phalanges, ow- 

 ing to the divergent direction of the digits when serving as a support. 

 In the distal end of the humerus, the same effect is seen, the external 

 part of the condyle nearly resembling the corresponding part of the meta 

 podial bones. This is traceable to the same cause, viz., the divergent posi- 

 tion assumed by the foreanu on the humerus, when the weight is sup 

 ported on one fore leg only. This brings the line of pressure through the 

 external part of both the head of the radius and the humeral condyle 

 (Fig. 42). 



Although I have already given what is essentially the same explanation 

 of this structure (op. cit., p. 199), the above renders clearer some points 



III. Atkophy op the Ulna and Fibula. 



Successive atrophy of the ulna and fibula is coextensive with reduction 

 of the number of the digits in tlie ungulate Mammalia, and with the de- 

 velopment of the digital patagium in the bats. This is in broad contrast 

 to the subequal development of the ulna and radius in the Cetacea, where 

 the fore limb functions as the blade of an oar. The cause of the re- 

 duction of the two elements in the Ungulata is the restriction of the func- 

 tions of the fore and hind limbs to the radius and tibia respectively. The 

 distal extremities of the ulna and fibula in primitive Ungulata were sup- 



