Once fairly started in life, these huge sharks spread themselves throughout the warm seas of the world, for there was none might stand before them and say nay. They swarmed along our southern coast, from Maryland to Texas; they swarmed everywhere that the water was sufficiently warm, for their teeth occur in Tertiary strata in many parts of the world, and the deep-sea dredges of the Challenger and Albatross have brought up their teeth by scores. And then--they perished, perished as utterly as did the hosts of Sennacherib. Why? We do not know. Did they devour everything large enough to be eaten throughout their habitat, and then fall to eating one another? Again, we do not know. But perish they did, while the smaller white shark, which came into being at the same time, still lives, as if to emphasize the fact that it is best not to overdo things, and that in the long run the victory is not _always_ to the largest. _REFERENCES_ _The finest Mosasaur skeleton ever discovered, an almost complete skeleton of Tylosaurus dyspelor, 29 feet in length, may be seen at the head of the staircase leading to the Hall of Paleontology, in the American Museum of Natural History, New York. Another good specimen may be seen in the Yale University Museum, which probably has the largest collection of Mosasaurs in existence. Another fine collection is in the Museum of the State University of Kansas, at Lawrence._ _The best Zeuglodon, the first to show the vestigial hind legs and to make clear other portions of the structure, is in the United States National Museum._ _The great sharks are known in this country by their teeth only, and, as these are common in the phosphate beds, specimens may be seen in almost any collection. In the United States National Museum, the jaws of a twelve-foot blue shark are shown for comparison. The largest tooth in that collection is 5-3/4 inches high and 5 inches across the base. It takes five teeth of the blue shark to fill the same number of inches._ _The Mosasaurs are described in detail by Professor S. W. Williston, in Vol. IV. of the "University Geological Survey of Kansas." There is a technical--and, consequently, uninteresting--account of Zeuglodon in Vol. XXIII. of the "Proceedings of the United States National Museum," page 327._ [Illustration: Fig. 12.--A Tooth of Zeuglodon, one of the "Yoke Teeth," from which it derives the name.] V BIRDS OF OLD "_With head, hands, wings, or feet, pursues his way, And swims, or sinks, or wades, or creeps, or flies._"