SKELETON. 59 



EPISTERNUM (INTERCLAVICLE) . 



In stegocephals and the oldest rhynchocephals there is a median 

 bone on the ventral surface, called the episternum (fig. 58). It 

 is rhomboid in front and may have a long posterior process, the medial 

 ends of the clavicles lying ventral to the broad anterior end. This 

 element is regarded as homologous with a T-shaped membrane bone 

 which occupies a similar position in lizards (fig. 56) and crocodilians, 

 where it acts as a brace between the shoulders. It arises by two centres 



FIG. 58. Shoulder girdles of (A ) Melanerpeton and (B) diagram of Branchiosaurus, after 

 Credner, the determination of elements after Woodward, cl, clavicle; co, coracoid; e, 

 episternum; s, scapula. 



of ossification in membrane and hence cannot be the same as the su- 

 prasternalia of mammals. An episternum also occurs in theriomorphs, 

 pythonomorphs, ichthyosaurs, and plesiosaurs, and possibly the 

 entoplastron of the chelonians (fig. 34, p. 42) is the same structure. 

 It has not been recognized in birds, but it reappears in the monotremes 

 among mammals (fig. 113), with nearly the same relations as in the 

 lacertilians. 



THE SKULL. 



The skull is a complex structure and the last word concerning its 

 composition has yet to be said. A century ago Oken pointed out that 

 a series of parts could be distinguished in the mammalian skull, each 

 of which somewhat resemble a vertebra in its general relations, and 

 thus laid a foundation for a 'vertebral theory of the skull' which was 

 farther developed by Owen. Huxley showed that these were superficial 

 resemblances, that the three or four vertebrae they would recognize were 

 nothing of the sort, and that the skull shows no real metamerism 

 except in the occipital region and in the visceral arches. 



In its development the skull, like the rest of the skeleton, passes 

 through two, and in the bony vertebrates, three stages: membranous, 



