Immense land reptiles (_Dinosauria_) as large as our largest mammals, and in some cases larger than the elephant, moved sluggishly over the land. Some walked on all fours; others were occasionally or usually bipedal, and walked upright like birds, and had many structural features in common with the latter; some were herbivorous, feeding on plants and even reaching into the branches of trees for their food, others were carnivorous, feeding on their fellow-creatures. Huge reptiles (_Ichthyosauria_) swam about in the sea in great numbers. Immense bat-like forms (_Pterosauria_) sailed through the air like birds, being literally flying dragons. These reptiles, which had branched off from the generalized Amphibians, were themselves very generalized creatures. It is interesting to remember that the Monotreme and Marsupial mammals found in this age were very low reptilian mammals; they were not typical, specialized Monotremes and Marsupials, like the modern creatures, but very generalized forms, being probably connecting links with low generalized Insectivora, which were the first of the true mammals. These early Monotremes and Marsupials were quite small animals, varying in size from a mole to a rabbit. They were insignificant creatures among the mighty giant reptiles, but they carried in their warm blood the promise of future mammalian supremacy. Birds appear for the first time in this period, but very different from modern birds. Not only did many of the reptiles of this time have bird-like characters, but all the birds of this period were distinctly _reptilian_ birds (_Archæopteryx_). Birds came off from primitive reptiles as branches from a stem (see Diagram of Development). It was only with the passing epochs that these early birds changed more and more from their reptilian characters and assumed more and more the features of modern birds. Many fossil connecting links between reptiles and birds have been discovered, and afford most useful illustrations of those changes in creatures that we call _Evolution_. =Cretaceous Period.= In the Cretaceous period the aspect of plant life has changed greatly. Now, for the first time, we meet with ordinary hard-wood trees, such as beech, oaks, hickory, maples, poplar, etc., but of very different genera and species from those existing to-day. In this period, for the first time, we find the highest order of Cephalopods, viz., the naked ones, allied to cuttlefishes and squids. They are known as the _Belemnites_. Sometimes the fossil ink-bags of these creatures are found so that they can be drawn in their own fossil ink. The Ammonites proper, which were introduced in the Triassic period and culminated in Jurassic time, become extinct at the close of the Cretaceous.