Triassic times saw the introduction of the Hexa coralla amongst the coelenterates, of special families of crinoids and the first introduction of the modern echinoids in the form of the diademoids derived from Cidaris, and leading on to all the other existing families. The brachiopods, although varied, belong to very few families and from this point onwards, except for the occasional abundance of individuals of a few species, play a sub ordinate part in the fauna. In association with this regression of the brachiopods we find an increased importance of the lamelli branchs, an increase which is paralleled and even exaggerated in the case of the gastropods; but the most characteristic Triassic molluscs are the cephalopods. During the Triassic times the ammonites became extraordinarily abundant, varied in type, and remarkably elaborate in structure. Some of them represent the culmination of families which had existed in Permian times, many originated in the Trias and did not survive that period, only one, the Phylloceratidae appearing first in the Upper Trias, lives on into the Jura to provide one of the two main stocks from which all other ammonites seem to have been derived. Concurrently the Nautiloidae exhibit a reduction in numbers and in range of struc ture, but the dibranchiate cephalopods, structurally the most advanced members of that group, first appear in the Trias in the form of a forerunner of the belemnites. Amongst the Crustacea the most important innovation was the introduction of the Deca poda, which have since become the leading group, whilst the single well-kdown Triassic insect fauna contains representatives of many families which are less archaic than those from the Carboniferous, leading up to the modern types.
The end of the Trias saw the disappearance of all the elasmo branchs except the Selachii, and the osteolepids, which do not survive the Permian, are represented only by their descendants the coelacanths. The Dipnoi are reduced to the single genus Ceratodus, which lived indifferently in fresh and salt water. Amongst the Actinopterygii the palaeoniscids, although still abundant, play a less and less important role, many families of "lepidosteoid ganoids" making their appearance and living on to become the predominant fish in the Jura. Three of the Palaeozoic orders of Amphibia had become extinct at the end of the Permian and the labyrinthodonts, although gigantic in size and world-wide in distribution, form a much less varied assemblage than their ancestors in the Permian. One family of them, however, took to the sea and provided the only known cases of Amphibia living in salt water.
During the Trias the Reptilia continue their evolution, the majority of the Permian orders living on in the form of advanced descendants, whilst new orders, the "dinosaurs" and Chelonia make their appearance on land and the seas are invaded by the unrelated groups of the ichthyosaurs, plesiosaurs and
Finally there is some evidence that the pterodactyls were already flying by the end of the Trias. But the most important single
event is the occurrence in the Upper Trias of mammals belonging to the sub-class Multituberculata.
The Jurassic fauna exhibits the appearance of a few families of Hexacoralla and of crinoids. In it the irregular echinoids make their appearance to undergo a rapid development in forms adapted to varied habitats, whilst the regular echinoids also branch out into new families. Amongst the brachiopods are found only the living families, together with three which died out in the Lias. Amongst the Bryozoa the now dominant group of Cheilostomata first appears. New families of lamellibranchs and gastropods were introduced, whilst the ammonites display an extraordinarily rapid evolution, the two fundamental stocks giv ing origin to thousands of species and hundreds of genera, abundant as individuals, sometimes wide-spread geographically but of very limited duration in time. The Dibranchiata were represented not only by the belmnites but also by forms which made a much closer approach to living Sepia. Amongst the arthropods, Brachyura (crabs) are added to the Macrura and amongst the insects such advanced types as the Diptera occur, together possibly with butterflies. The elasmobranchs are now entirely selachians and skate-like forms make their appearance, whilst the Holocephala become comparatively abundant and soon obtain a structure which is in all essentials identical with that of the living members of the group. Amongst the bony fishes the coelacanths live on without alteration of their structure, and the Dipnoi leave the seas of the world. Amongst the Actinopterygii the palaeoniscids are rare and little varied, whilst both salt and fresh water were occu pied by abundant and very varied lepidosteoids, together with a few species, but abundant individuals, of a group Leptolepidae apparently ancestral to the Teleostei, which includes the vast majority of recent fish. Only two Jurassic Amphibia are known, a typical frog and a newt. The reptiles were still a predominant group of vertebrates, but many of the Triassic orders had become extinct, and the real range and structure which they exhibit is much smaller. The "dinosaurs" flourished on land in all parts of the world and were accompanied by tortoises, Rhynchocephalia and lizards. Pterodactyls almost as uniform in structure as the birds of to-day occupied the air and seem to have led a life similar to that of a gull. The ichthyosaurs and plesiosaurs are joined in the ocean by crocodiles and turtles. In all habitats gigantic forms appear, paralleling roughly the adaptive develop ments later made by mammals. The first birds are represented by two skeletons of a primitive structure from the Lithographic stone of Bavaria. The Jurassic mammals are all small, none having a skull exceeding three inches in length, but they belong to three sub-classes, one of which is possibly related to the ancestry of the living marsupials.