THE MAMMAF.IA OP THE UIN'TA FORMATI()>f. 561} 



5. We are indebted to Prof. Harrison Allen's* studies of the eelebi'ated Muv- 



« 



bridge i)hotograiAs for the following laws of quadrupedal motion : As the fore foot 

 descends it passes under the body (p. 51), i.e., inwards; striking the ground, it 

 arrests the torsion of the trunk as the shoulder is turning forwards towards the oppo- 

 site side (p. 57) ; it strikes upon the outer border (p. 8S) and leaves by the inner 

 border (p. 50), so that the pressure of the body is boine from without inwards across 

 the foot; as the foot is raised the sole is everted (p. 50). It follows [nobis) that to 

 strike upon the outer border the sole must be slightly inverted as the foot is lowered; 

 secondh', in so far as the trunk torsion is transmitted to the feet, the feet must be 

 rotating upon their own axis from the outer to the inner sidfe. Let us first see how 

 this theory of foot motion agrees with the metapodial displacement. 



G. Tlie metapodial displacement is pi'actically uniform throughout the Ungulata 

 in the carpus, but very diverse in the tarsus. This difference is undoubtedly duL- to 

 the fact that the fore foot is for support, the hind foot for propulsion. The latter 

 moves forwards and backwards in a nearly straight line, thus the meta])odials are 

 vai-iously displaced in accordance with slight lateral variations in this orthal move- 

 ment. Jn tlie manus the second and third metacarpals invariably have ectal facets 

 upon the magnum and unciform lespectively. The only variation is in the adaptively 

 reduced Artiodactyla in which the third metacarpal spreads also upon the trapezoid 

 (Diag. 9). This disposition of the facets is perfectly adapted to resist the strain 

 upon the metapodials as the foot swings inward in. descent, this motion being univer- 

 sal among the primitive types. Thus the ectal displacement of the metapodials be- 

 gins with the elevation of the wrist joint and precedes the inteicari)al dis})lacement 

 (see Phenacodus, Diag. 8). It does not advance but appears to he retaided by the 

 elongation of the metapodials, as in the pala^othere and equine line, as tin; lateral motion 

 diminishes. The extension of these carpo-metacarj)al facets is largely a matter of 

 reciprocal growth, i.e., the unciform, for example, extends inwards as the thiid ineta- 

 cari)al extends outwards (compare Diags. 8, i<). 



7. Growth of the scaphoid and unciform. As a result of metacarpal displace- 

 ment and of digital reduction, the unciform, already supporting the tourlh and lillh 

 digits, received a portion of thi' third digit. As the lil'tli digit persisted longer than 

 the first, both the motion of the foot striking upon the outi'i- i)order and the trans- 

 mission of the direct weij^ht throu<rh three diyfits contributed the maxiiuum vertical 

 and lateral strain to tlie ectal side of the second row of carpals and especially to the 

 unciform, while in the bones of the first row the cntal side, the scaphoiil and lunar, 



•The Muyljridgc Work al tlic Uiiivi-rsily of Pennsylvunlii. MiiluriitlM lor a .Mciimir u|ion Aniin^il [.■!■ .uni.iiriii. Ily 

 Hnrrison Alkii. I'hila., 188H. 



