These animals of the Upper Cretaceous are probably descended from the Iguanodonts of an older period. But the long ages that intervened, some millions of years, have brought about various changes in the race, not so much in general proportions as in altering the form and relations of various bones of skull and skeleton and perfecting their adaptation to a somewhat different habit of life, so that they must be regarded as descendants perhaps, but certainly rather distant relatives, of the older group. We know more about the Trachodonts than any other dinosaurs. For not only are the skeletons more frequently found articulated, but parts of the skin are not uncommonly preserved with them, and in one specimen at least, so much of the skin is preserved that it may fairly be called a "dinosaur mummy." This specimen of _Trachodon_ is in the American Museum, and beside it are two fine mounted skeletons of the largest size. There is also on exhibition a panel mount of a nearly related genus, _Saurolophus_ the skeleton lying as it was found in the rock, and a fine skeleton of a third genus _Corythosaurus_ with the skin partly preserved on both sides of the crushed and flattened body stands beside it. In the _Tyrannosaurus_ group when completed will appear a fourth skeleton of the _Trachodon_. Several skulls and incomplete skeletons on exhibition and other skeletons not yet prepared add to the Museum collection of this group. Trachodon skeletons may also be found in the Museums of New Haven, Washington, Frankfurt-on-the-Main, London and Paris, but nowhere a series comparable to that displayed at the American Museum. THE TRACHODON GROUP. The following description of the Trachodon group is by Mr. Barnum Brown and first appeared in the American Museum Journal for April 1908:[16] "This group takes us back in imagination to the Cretaceous period, more than three millions of years ago, when Trachodonts were among the most numerous of the dinosaurs. Two members of the family are represented here as feeding in the marshes that characterized the period, when one is startled by the approach of a carnivorous dinosaur, Tyrannosaurus, their enemy, and rises on tiptoe to look over the surrounding plants and determine the direction from which it is coming. The other Trachodon, unaware of danger, continues peacefully to crop the foliage. Perhaps the erect member of the group had already had unpleasant experiences with hostile beasts, for a bone of its left foot bears three sharp gashes which were made by the teeth of some carnivorous dinosaur. [Illustration: Fig. 28.--Mounted Skeletons of _Trachodon_ in the American Museum. Height of standing skeleton 16 feet, 10 inches.]