474 CHORDATA. 
crurotarsal joint, whereas in reptiles and birds the main 
joint is an intertarsal joint. 
In both limbs of mammals the number of phalanges is 
normally two in the first digit and three in each of the 
others. 
In the various orders we shall notice that there may 
occur fusions of certain bones, loss of others and modifica- 
tions of others, but when once this type be learnt and 
retained in one’s mind, there is no difficulty in interpreting 
aright the most modified mammalian limb. 
As general rules for the identification of the bones we may lay down 
the following (see Fig. 326) :— 
1. Humerus and Femur.—The proximal limb-bones (Aumerus and 
Jemur) are long bones and have az articular condyle at each end. 
Towards their proximal ends they have a ‘‘ ball” which moves in the 
socket of the limb-girdle, and two or more processes called /zcberostties 
(humerus) or ¢vochanters (femur). At the distal extremity they both 
have a sigmoid condyle or ‘¢vochlea. The humerus may be distin- 
guished from the femur by its large shallow condyle, whereas the femur 
has a rounder condyle raised on a ‘‘ neck.” The humerus usually has 
a conspicuous deltoid ridge on its preaxial border. The proximal 
end of the humerus (or femur) can always be distinguished from the 
distal by the condylar or ball-and-socket joint in the former and the 
sigmoid or é2/atera/ joint in the latter. 
The humerus often has a small foramen on the inner or postaxial 
side of the sigmoid condyle termed the entepicondylar foramen. It 
seems to have occurred very generally amongst Eocene mammals, such 
as Condylarthra, Tillodontia and Typotheria, and is very generally 
found amongst Metatheria, Edentata, some Carnivora, most /nsectivora, 
Lemuroidea and Cebide. 
This foramen should be carefully distinguished from the supra- 
trochlear foramen in the median line above the trochlea and produced 
by incomplete ossification. 
The ¢hird trochanter of the femur has much the same interest as the 
entepicondylar foramen. It is on the postaxial border (cf Horse) 
and is present in Condylarthra, Tillodontia, Typotheria, Creodonta and 
other extinct types. It also occurs in Dasyfodide, Orycteropodide, 
many Rodentia, most Iusectivora, in Pertssodactyla and (small) in 
Hyracoidea. 
z. Distal limb-bones.—The distal limb-bones have a hollow artz- 
cular facet at each end when they are fully developed. At the 
proximal extremity they receive the condylar ends of the proximal 
limb-bones ; at the distal end they receive the condyles of the proximal 
carpals. 
The two most important bones are the preaxial (or the radius and 
zibza), and the ulna and fibula are, in a great number of cases, merely 
vestiges fused on to their respective preaxial bones, forming a single 
