THE SKELETON OF THE FROG. PELVIC GIRDLE. 165 



carpale 1 with both the first and second. The third bone is 

 large and articulates with the third, fourth and fifth meta- 

 carpals, it represents carpalia 3 5, with probably in addition 

 the representative of a second centrale. 



The hand consists of four complete digits, and a vestigial 

 pollex reduced to a short metacarpal. 



Each of the four complete digits consists of a metacarpal 

 and a variable number of phalanges. The first digit, as just 

 mentioned, has no phalanges, the second and third have two, 

 and the fourth and fifth have three. 



C. THE PELVIC GIRDLE. 



The pelvic girdle of the Frog is much modified from the 

 simple or general type found in the Newt (p. 149). 



It is a V-shaped structure consisting of two halves which 

 are fused together in the middle line posteriorly, while in 

 front they are attached to the ends of the transverse pro- 

 cesses of the sacral vertebra. Each half bears at its posterior 

 end a deep cup, the acetabulum, with which the head of the 

 femur articulates. 



Each half of the pelvis ossifies from two centres. The 

 anterior and upper half of the acetabulum, and the long 

 laterally compressed bar extending forwards to meet the sacral 

 vertebra ossify from a single centre and are generally called 

 the ilium ; it is probable however that they represent both 

 the ilium and pubis of mammals 1 . The posterior part of 

 this bone meets its fellow in a median symphysis. 



The posterior third of the acetabulum is formed by a small 

 bone, the ischium, which likewise meets its fellow in a 

 median symphysis. 



The ventral portion of the pelvic girdle never ossifies, even 

 in old animals being formed only of calcified cartilage. This 

 is generally regarded as the pubis, but it perhaps corresponds 

 to the acetabular bone of mammals. 



1 See bottom of p. 187. 



