ii6 
The An 
Naturalist. 
[March, 
pinching of a keel by its groove takes place at all points in 
the length of the former reached by the opposite sides of the 
extremities of the latter during flexion and extension (fig. 
14). This keel becomes acute and prominent in the 
Booidea, and extends to the anterior face of the condyle 
fig. II, Cosoryx fiircatus). This development has been 
apparently especially due to the presence of two sesamoid 
bones, embedded in the flexor tendons, one on each side 
of the middle line of the posterior side of the metapodial con- 
dyle. The fissure between these bones has permitted the 
moulding of the surface into a keel to fit it. That this has 
been the case is further indicated by the fact that a single me- 
dian trochlear surface exists at the distal extremity of the first 
phalange in all mammals. But a single flexor tendon crosses 
Fig. 14 — Tongue and groove joints in Cerz'ns elaphus. 1-3, elbow j 
trochlear keel and groove. 1-2, in place ; 3. radius dislocated by extern: 
4-5, metatarso-phalangeal articulation; 4, in place; 5, dislocated by I 
phalange ; D P, the dead or fixed point. 
