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 
Fic. 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 vertebre 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, 
