Perhaps nowhere in the world are the fossil remains of marine animals more abundantly and better preserved than in these famous chalk deposits of Kansas. The exposures are of great extent—hundreds of square miles—and the fossil treasures they contain seem inexhaustible. Long-continued explorations by collectors have brought to light thousands of specimens of these swimming lizards, some of them of extraordinary completeness and perfect preservation, so complete and so perfect that there is scarcely anything concerning the mosasaurs which one might hope to learn from their fossil remains that has not been yielded up by these many specimens. The complete structure and relations of all parts of the skeleton, impressions of the bodies made in the soft sediments before decomposition had occurred, the character of their food, the nature of the skin covering, and even some of the color markings of the living animals have all been determined with certainty. Not only from Kansas, but also from many other parts of the world, have remains of these animals been discovered, until now it may truthfully be said that no other group of extinct reptiles is better represented by known fossil remains than the mosasaurs. From England, Belgium, Russia, and France in Europe; from New Jersey, Georgia, Alabama, Mississippi, Texas, New Mexico, Colorado, Kansas, Nebraska, the Dakotas, Wyoming, and other places in the United States; from New Zealand and South America they have been obtained in greater or less abundance and perfection. [Illustration: FIG. 70.—Tooth of _Tylosaurus_.] Their geological history is relatively brief, notwithstanding their wide distribution over the earth in such great numbers and diversity. The earliest are known from near the beginning of the Upper Cretaceous of New Zealand, whence it is believed by some that they migrated to other parts of the world, appearing in North America some time later. They reached their culmination in size, numbers, and variety very soon, and then disappeared forever before the close of Cretaceous time. The largest complete specimen of a mosasaur known measures a little more than thirty feet in length, but incomplete skeletons of others indicate a maximum length of about forty feet. The skulls of the largest species are about five feet long. The smallest known adult skeletons are scarcely eight feet in length. There are now known at the present time seven or eight genera of three distinct types, all belonging to one family, the Mosasauridae, including about twenty-five known species. While a few of the genera are widely distributed over the earth, the species are all of restricted range, indicating, perhaps, non-migratory habits.