QAAAA. Scapular arch external to ribs; two longitudinal post-orbital bars (paroccipital arch distinct) (Archosauria). a. A supramastoid bone. Ribs two-headed ; a clavicle and interclavicle ; acetabulum closed; no obturator foramen; ambulatory; vertebrae amphicoelous. Order 7, PELYCOSAURIA. aa. No supramastoid. 1 Proc. Amer. Assoc. for the Advancement of Science, loth meeting (Cambridge, 1871, 8vo), pp. 230 sq.; Amer. Naturalist (1889), vol. xxiii. p. 863. 2 Syllabus of Lectures on the Vertebrata (Philadelphia, 1898, 8vo), P. 54.141 Ribs two-headed; interclavicle not distinct; external digits greatly elongated to support a patagium for flight. Order 8, ORNITHOSAURIA. Ribs two-headed; no interclavicle; acetabulum open; ambulatory. Order 9, DINOSAURIA. Ribs two-headed; an interclavicle; acetabulum closed; ambulatory. Order 1o, LORICATA. Ribs one-headed; an interclavicle; acetabulum closed, a large obturator foramen; ambulatory. Order 1I,