THE SKELETAL SYSTEM. 61 
are considered individually. The corresponding portions may be 
identified (Fig. 32), though in mammals the ventral portion in the 
case of the pectoral girdle is greatly reduced. Of the ventral 
elements, those commonlypresent in vertebrates are the coracoid, 
GROUND PATTERN which forms the posierior ray, and the 
IN TERRESTRIAL procoracoid, the anterior ray, the latter 
VERTEBRATES. being partly covered in front by a 
derm splint, the clavicle. This condition 
though not characteristic of mammals generally is_ still found in 
monotremes, and rudiments of the coracoid extension ventrally are 

A in mt Wwe 
of iit 
Fé if \\) 
1 } tg | p44 | 
| | Sais, ( 4 \ 
| te H\ | W \\ 
| @ v@ \4 
Ko H i I 
aes | ' \ re) 
vy af 
A B Cc 
Fic. 33. Homologies of the mammalian limb. A, fore foot, rabbit 
B, fore foot, horse. C, human hand. r, radius; u, ulna; I-V, metacarpa 
bones. 
identifiable in embryonic marsupials. Adult marsupials and 
placentals show only a small hook-like coracoid process, together 
with the clavicle in perfect or less perfect development. 
There is no more striking feature of homology than that shown 
by the free extremities in respect of the different forms of vertebrates. 
This is true homology because it concerns the resemblances, part 
for part, in the anterior or in the posterior limb of any one verte- 
brate as compared with the corresponding elements in the same 
position in other forms. The front limb of the rabbit (Fig. 33) is 
slightly elongated, and semi-digitigrade, making it a more efficient 
