The oldest Ornithosaurians are from the Muschelkalk of Germany. In England the oldest are from the Lias,--several species of Dimorphodon--a genus in some respects nearly resembling the Pterosaurians of the Cambridge Upper Greensand. In the Oolite of Stonesfield are several species of Rhamphorhynchus or a similar genus. The great Pælolithic period from the Oxford Clay to the Kimeridge Clay, has yielded in its several divisions small Pterodactyles of new species. And the Psammolithic period from the Portland Sand to the Lower Greensand has afforded many excellent remains both of true Pterosaurians in the Purbeck, Wealden, and Potton Sands, and of animals which indicate a new order of Ornithosauria having affinities with Von Meyer's thick footed saurians, the Dinosauria. In the Cretaceous series, Galt, Upper Greensand, and Chalk all have representatives of the Pterosauria; but no English stratum has hitherto yielded so many as the Cambridge Upper Greensand. From this formation the collection accumulated during Prof. Sedgwick's long professoriate is unequalled; though, excepting a few fine bones from the Chalk and the Purbeck Limestone, the Woodwardian Museum is as yet deficient in Ornithosaurians from the other Secondary Rocks. Until descriptions of these animals shall have been published a classification of the Ornithosauria must necessarily be provisional. And it cannot be expected that descriptions of the structure of Cretaceous Pterosaurians here given will hold good for all the Ornithosaurian sub-class. Finally, I have gratefully to express my thanks to the many friends, English and German, who have aided me with specimens and with their writings; to the chiefs and officers of the English museums, especially Prof. Owen, Prof Humphry, Prof Newton, Prof Phillips, Prof Flower, and Prof. Huxley; to the officers of the University Library, especially Mr Bradshaw, and Mr Crotch, for aid in consulting books; but chiefly to Prof Sedgwick, who while employing me as his paid Assistant to aid him in his Museum work, has generously encouraged me to carry on for several years, without restraint and as part of my daily labour, an investigation of which this treatise is the first fruit. Prof. Sedgwick has placed at my disposal an ample number of copies for distribution among those who take an interest in the Museum; and especially among those who have contributed to the Ornithosaurian collections, and aided me in my work. _January 3, 1870._ CONTENTS.