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 formations, but all the Jurassic and some of the Cretaceous genera have the secondary bony plate less extended backwards than that in the Tertiary and existing genera, while their vertebrae have flattened or concave ends, instead of exhibiting a balland-socket articulation. Some of the Upper Jurassic crocodiles (Metriorhynchus) were more truly aquatic than any now living, with the fore limbs degenerate, the hind limbs much enlarged for swimming, and the dermal armour lacking. The end of the vertebral column is bent downwards, as in Ichthyosaurus, so they doubtless possessed a similar triangular tail-fin. Typical crocodiles and alligators date back to the close of the Cretaceous period, and they did not become extinct in Europe until the beginning of the Miocene period. Remains of an extinct alligator (Diplocynodon) are common in the Upper Eocene sands of the Hordwell cliffs, Hampshire.

9.—Pterodactylus spectabilis, natural size, from the Lithographic Stone. h, humerus; ru, radius and ulna; mc, metacarpals; pt, pteroid bone; 2, 3, 4, digits with claws; 5, elongated digit for support of wing-membrane; st, sternum, crest not shown; is, ischium; pp, prepubis. The teeth are not shown. (After H. von Meyer.)

Order 8. —The flying reptiles or Pterodactyls (fig. 9) are completely evolved at their earliest known appearance in the Lower Lias (Dimorphodon), and exhibit little essential change as they are traced upwards through the Mesozoic formations. The latest Cretaceous genera, however, comprise the largest species, which have been found in Europe, N. America and Brazil. Some of these (Pteranodon) are toothless, and their wings are so large that for adequate support the pectoral arch is fixed to the vertebrae like a pelvis. The wings occasionally have a span of from 5 to 6 metres. The wing-membranes are only known in the European Jurassic genus, Rhamphorhynchus (fig. 10), found well preserved in the fine-grained lithographic stone of Bavaria. In this genus there is also a rhomboidal flap of membrane at the end of the tail.

10.—Rhamphorhynchus phyllurus, from the Solenhofen Lithographic Stone, about ¼ natural size, with the greater part of the wing-membranes preserved. x, caudal membrane; st, sternum; h, humerus; sc, scapula and coracoid; wm, wing-membrane. (After O. C. Marsh.)

Order 9. —The ancestors of the lizards and snakes can only be traced back definitely to the latter part of the Cretaceous period. They were then represented by two suborders of aquatic reptiles, the Dolichosauria and Pythonomorpha (or Mosasauria), which are in many respects intermediate between the existing Lacertilia and Ophidia. The Dolichosauria, from the Upper Cretaceous of Europe, are small and snake-like

in shape, but with completely formed limbs. The Pythonomorpha are known from Europe, N. and S. America and New Zealand, and sometimes attained a very large size, the typical Mosasaurus camperi from Maastricht being about 15 metres in length. Their limbs are powerful paddles. Their trunk and tail are often much elongated, so that their shape is snake-like, as shown by Clidastes (fig. 11), from the Chalk of Kansas, U.S.A. The Lacertilia and Ophidia, so far as known, are exclusively Tertiary and Recent reptiles. Marine snakes (Palaeophis) occur in the Eocene of the London and Hampshire basins.

.—General Works on Extinct Reptiles.—K. A. v. Zittel, Handbuch der Palaeontologie, vol. iii. (Munich, 1887-1889).—H. A. Nicholson and R. Lydekker, Manual of Palaeontology, vol. ii. (Edinburgh, 1889).—R. Lydekker, Catalogue of the Fossil Reptilia and Amphibia in the British Museum, vols. i.—iv. (London, 1888-90).—A. S. Woodward, Outlines of Vertebrate Palaeontology (Cambridge, 1898).—K. A. v. Zittel, Text-book of Palaeontology, ed. C. R. Eastman, vol. ii. (London, 1902). Anomodontia: R. Owen, Catalogue of the Fossil Reptilia of South Africa in the Collection of the British Museum (London, 1876).—E. D. Cope, “The Reptilian Order Cotylosauria,” ''Proc. Amer. Phil. Soc.'' vol. xxxiv. (1896), p. 436, and vol. xxxv. (1896), p. 122.—E. T. Newton, “Some New Reptiles from the Elgin Sandstones,” ''Phil. Trans., vol. 184 (1893), p. 431.—Various papers by R. Owen in Quart. Journ. Geol. Soc.'', 1876-1884, by H. G. Seeley in ''Phil. Trans.'' (1889-1895), and by R. Broom in ''Proc. Zool. Soc., Ann. S. African Museum and Trans. S. African'' ''Phil. Soc.'' (from 1900 onwards). Chelonia: G. Baur, “Bemerkungen über die Phylogenie der Schildkröten,” ''Anat. Anzeiger'', vol. xii. (1896), p. 561.—Technical papers by F. A. Quenstedt in Württ. Jahresh. vol. xlv. (1889), p. 120 (Proganochelys).—G. R. Wieland in ''Amer. Journ. Sci.'' ser. 4, vol. ii. (1896), p. 399 (gigantic Cretaceous leathery turtle), and E. C. Case, ''Journ. Morphol.'' vol. xiv. (1897),