RhynchocephalaRhyn`cho*ceph"a*la\, n. pl. [NL., fr. Gr. "ry`gchos snout + kefalh` head.] (Zo["o]l.) An order of reptiles having biconcave vertebr[ae], immovable quadrate bones, and many other peculiar osteological characters. Hatteria is the only living genus, but numerous fossil genera are known, some of which are among the earliest of reptiles. See Hatteria. Called also Rhynchocephalia.