THE FROG 43 
The pubis is a triangular plate of cartilage, more or !ess cal- 
cified, wedged in between the ilium and ischium ventrally and 
forming about one-sixth of the acetabulum. The two pubes are 
united by the whole of their median surfaces, forming the pubic 
symphysis. 
Draw (X 4) the pelvic girdle viewed from the lateral aspect. 
Compare the pelvic girdle and its relation to the vertebral 
column in the frog with the corresponding features in a tailed 
amphibian (urodele). In the phylogenetic history of the frog 
the fusion of the posterior vertebrae to form the coccyx was 
doubtless associated with a forward migration of the attachment 
of the pelvic girdle, and the elongation of the ilia. The urodele 
presents the more primitive condition, and the peculiar structure 
of this part of the skeleton of the frog is the result of specializa- 
tion correlated with its leaping habits. 
Examine the pelvic girdle in skeletons of the turtle and of man. 
Obviously the pectoral and the pelvic girdles are built on es- 
sentially the same plan. Structures that are metamerically re- 
peated are said to be serially homologous. What element in the 
pectoral girdle is homologous with each of the elements of the 
pelvic girdle? Better examples of serially homologous structures 
are found in the vertebral column, and in the limbs, 
4. The hind-limb. The bones have the same general character 
as those of the fore-limb, with which they are homologous. 
The thigh. The femur or thigh-bone is a long bone with a 
slight sigmoid curve. The proximal end or head is spheroidal 
and fits into the acetabulum, forming the hip-joint; the distal end 
is somewhat broadened for articulation with the tibio-fibula. 
The leg or crus. The tibio-fibula is slightly longer than the 
femur, slightly curved, and broadened at both ends. Like the 
radio-ulna, this is a compound bone, formed of two bones usually 
separate in other animals (compare the human skeleton). Near 
each end, a longitudinal groove indicates the line of fusion of the 
two bones. 
The ankle or tarsus contains five tarsal bones. The two 
proximal ones are long bones united at each end by common 
epiphyses. The preaxial one is known as the tibiale or astrag- 
alus, the postaxial and larger one is the fibulare or calcaneum. 
4 
