Page:The Last link.djvu/75

Rh double (such a condylar arrangement begins with the Amphibia, but only the two lateral condyles are developed, while the middle portion, belonging to the basi-occipital element, remains rudimentary ); (4) The formation of centra, or bodies of the vertebræ, mainly by a ventral pair of the original quadruple constituents, or arcualia.

17. Between the Proreptilia and the Mammalia, which latter occur in the Upper Triassic epoch, we have necessarily to intercalate a group of very low reptiles, which are still so generalized that their descendants could branch off either into the Reptilia proper or into the Mammalia. The changes concerned chiefly the brain and the heart; of the skele-