1889.1 The Artiodactyla. 115 
mammals. (Plate IV. figs. A, D, Hyaena, Eucrotaphus.) 
With the compression of the external part of the condyle, the 
external slope becomes steeper and is at length nearly verti- 
cal (Ibid, fig. E, Cervus). The mechanical cause of this troch- 
lear crest is the use of a single fore leg to support the body in 
rapid locomotion. As had been remarked by H. Allen, a mod- 
ern Artiodactyle in rapid motion lights on one forefoot, which 
strikes the earth immediately on or even beyond a point 
below the middle of the body (fig. 13). This throws the im- 
pact principally on the external side of the humeral condyles, 
with the result stated. A similar cause produces a similiar 
result in the development of the tongue and groove articula- 
tion between the metapodials and first phalanges. In light- 
ing on a didactyle foot, the toes are naturally spread, the 
result being to throw both 
vay 
axis ot impact, in diver- 
gent directions. The re- 
sult of this impact is to 
produce on each metapo- 
dial condyle as in the case 
of the humerus, an exter- 
nal roller of smaller diam- 
eter than the rest of the 
condyle (fig. 11), and sep- 
arated from it by an ab- 
rupt crest. Inboth humer- 
us and metapodial bones 
these crests are accentu- 
ated by a pinching process 
during flexion and exten- 
sion. This is produced 
by the longitudinal tor- 
sion which results in all 
limbs in motion from the 
arrest of the outward 
rotation of the foot by the 
earth, on alighting. The 
