saurs, and the fact that marine saurians had scale-covered skins. The paddle was formerly a matter of conjecture, and, in the absence of such remains, the saurians were supposed to have had marine turtlelike flippers. Prof. O. C. Marsh, of Yale College, was the first naturalist to discover sufficient of the missing parts of skeletons to determine that marine saurians propelled themselves with paddles rather than flippers. As to the scales and skin found perfectly preserved by Snow, they do not differ materially from those of the Old World lizards, the monitors, existing to-day. The paddles, skin, and scales are very delicate functions, and it is remarkable that they should have been preserved through millions of years. Williston says of the paddles: "The specimen figured by Chancellor F. H. Snow, of the University of Kansas, has been thoroughly cleaned from the matrix, enabling an accurate drawing to be made, also a photographic reproduction as it lies on a chalk slab. The parts concealed beneath the ribs and vertebræ have been carefully laid bare from the opposite side and their position shown. The position of the paddle is a natural one, and the fact is of interest as showing the general expansion and curvature of the digits." The limb is very flexible, with considerable space between the bones, which were but partly filled out with cartilage, and must have had very free articulations. The remains of the skin were found between the bones, indicating a thin, pliable membrane, and extending fully between the fingers to their tips. Small scutelike scales extended as far as the metacarpals. The fifth finger is long. The paddles are slenderer, more flexible, and relatively longer than in other genera, which, with other characteristics, would show that Tylosaurns was the least lizardlike of the Pythonomorpha (Cope). As to the structure of the hind paddle, it is of interest in having five functional toes, although Williston thinks that the fifth toe was undergoing reduction, and that the first toe was not as long as in the front paddle. He concedes five toes to the hind paddle of Platecarpus (Cope), but doubts, in the absence of a complete skeleton, if Clidastes had more than four functional toes, as in Mosasaurus. Upon this character, together with the absence of a sternum, he has established two families, Tylosauridæ and Mosasauridæ, and the two typical genera, representing the extremes of development of this order of reptiles.
Mosasaurs are known to have existed in many parts of the world, New Zealand, North and South America, and Europe, the oldest being regarded as the New Zealand types, Liodon and Taniwhasaurus. Dollo thinks that New Zealand was the center of their irradiation, where they appeared in the end of the Cenomanian,[1] to appear in America in the Turonian,[1] whence they migrated to
