FINS AND LIMBS 



3i7 



of a dorsal scapula and a ventral coracoid. In the Theromorph 

 reptiles the scapular girdle may have two ventral elements, 

 the coracoid and precoracoid. In the pelvic girdle the ilium 

 becomes attached to the sacral vertebrae, and the ischio-pubic 

 foramen appears between the pubis and ischium. In some 

 Dinosaurs a post-pubis is present, extending back beneath the 

 ischium. In Chelonia, the pectoral and pelvic girdles have a 

 peculiar position in that they lie inside the ribs, instead of 

 outside them as in other forms. In birds, the pubis rotates 



Fig. 162. — Diagrams illustrating the evolution of the limbs of Tetrapods. 



A and B, views of the early stage when the limbs stick out laterally and 

 the ventral surface of the body rests on the ground. C and D, later stage, 

 when the body is lifted off the ground, the forearm and shank being vertical, 

 and the limbs projecting to the side. E and F, late stage, when the hind 

 limb is rotated forwards from the acetabulum, and the fore limb rotated 

 backwards from the glenoid cavity ; but the hand points forwards and the 

 radius and ulna are crossed. 



backwards and comes to lie parallel to and beneath the ischium, 

 with which it may to a certain extent fuse. 



In mammals, the coracoid, precoracoid, and interclavicle 

 are retained only in the Monotremes. The pelvic girdle of 

 Monotremes and of Marsupials is characterised by the presence 

 of a pair of epipubic bones, which support the pouch, or marsu- 

 pium. The clavicle is often missing in the higher mammals, 

 and especially those which use their limbs for fast running. 

 So the clavicle is absent in the horse, and it is much reduced 



