Since the reptiles of the Mesozoic era constitute one of the few most remarkable and diversified classes of animals which ever inhabited the earth, we shall attempt to give the reader a fair idea of the most typical groups which have been totally extinct since the close of the Mesozoic era some millions of years ago. Of the swimming reptiles which lived in the seas many types are known and only a few will be described. Among these one important type was the ichthyosaur, a fishlike form which not uncommonly grew to be twenty to even forty feet long (Plate 18). The large head, sometimes four or five feet long, contained as many as 200 big sharp teeth and enormous eyes up to a foot in diameter. The body was heavy set, and the neck very short. There were four short, stout swimming paddles, and the tail was vertebrated. Some specimens of ichthyosaurs have been so perfectly preserved in Mesozoic strata that even the unborn young are plainly seen in the bodies! In some cases it is actually possible to tell what was the last meal of a particular ichthyosaur those millions of years ago; in one specimen, for example, remains of 200 creatures of the "cuttle-fish" tribe having been found in the exact position of the stomach. [Illustration: Fig. 59.--Chart showing the main branches in the history of Vertebrate (back-boned) animal life reaching its culmination in man. (By the author, in part after Cleland.)] The mosasaurs of the late Mesozoic were the only real sea serpents of the geologic ages. They were something like the ichthyosaurs, but with smaller heads and much longer, more slender, serpentlike bodies. Some grew to be thirty or forty feet long. Plesiosaurs were perhaps the strangest of all the Mesozoic marine reptiles. They grew to be forty to fifty feet long, with stout body, very long, slender neck, small head, short tail, and four long, powerful swimming paddles which were distinctly leglike. These and the mosasaurs were both flesh eaters, as shown by the sharp teeth. [Illustration: Plate 15.--(_a_) Restoration of a Late Paleozoic (Coal Age) Landscape. Showing the main kinds of plants which have entered into the making of most of our coal. Giant "club mosses" both with and without branches, in the left background; giant "horsetail" (or "scouring rush") plants on the right; and seed ferns in the left foreground. A primitive reptile in the water; two large amphibians or giant salamanders, called "stegocephalians," on the land; and a great "dragon fly," two feet wide, in the air. (_From a drawing by Prof. Williston. Courtesy of D. Van Nostrand Co._)] [Illustration: Plate 15.--(_b_) Photograph of a Fossil Fern or Seed Fern Frond on a Piece of Shale Millions of Years Old. The specimen is of the Pennsylvanian Age and was taken from the coal fields of Pennsylvania. (_After White, U. S. Geological Survey._)]