The Artiodactyla. 
113 
take place, the t/pe became extinct, as in Elotherium. He 
supposes that the extinction of such types was due to the 
feebleness of the latter construction, which precluded the 
attainment of any considerable speed on the part of its pos- 
sessor. The types in which this expansion took place per- 
sisted, and 
became 
the 
ncestors 
of the existing forms. 
As an example, see 
Procamelus. 
(Fig. 10.) 
The specialization 
of the elbow joint first 
^ \ 
becomes pronounced 
1 
in the Artiodactyla in 
c„ 1 
the Tragulidae. This 
/tf^s 
consists in the devel- 
fi-S,\ 
opment of the external 
M 
ifpCd 
part of the condyles of 
^"^^ 
the humerus into a 
V 1B 
// 
roller of contracted 
\v'J5 
diameters, and sepa- 
^r^/'^ 
rated from the remain- 
-m^ 
' 
ing part of the con- 
dyles by a keel, or 
tongue. The roller 
and tongue work into 
«'"^-^^^JB.«H 
/ l«l \ 
a corresponding plane 
mec ,'Tf^™f^^ 
/ Uj 
and groove of the head 
]V*l'.;r,i| 
' " 1 
vTl 
of the radius, forming 
' • ' 
1 u i 
an interlocking joint 
fM«\ 
of great strength. The 
(/ 
fM 
1 ? T 1 
strength of the union 
/I' 
f^ \^ 
between the radius 
\ ^ 
W 
V 1 \ 1 
and the ulna is in- 
^a:^-^ 
M V 
creased by the devel- 
opment of a keel on 
A. 
B. 
the inferior side of the 
Fig. 9. Pes of Artiodactyla 
A. 
Merycochoeru 
head of the former. 
montanus Cope, two-fifths n 
taurns. L. one-fourth natur 
!'S^ 
size. ^.Bo 
which fits a groove on 
the upper side of the 
latter. Both of thes 
uctures ca 
be traced from their be- 
ginnings in 
the Artiodac 
t> 
la. (Pla 
te V.) 
