I ntroductory N ote . The remains of Dinosaurs were for many years very rarely met with in the Cambridge Upper Greensand; and several important parts of the skeleton have never yet been found. But a considerable collection of more than 500 bones is now preserved in the Woodwardian Museum alone; and since the greater number of these fossils have been discovered in larger or smaller sets of naturally associated remains, each of which is obviously a portion of the skeleton of a single individual, they afford evidence on which it is possible to establish many species which belong to several genera. Occasionally the series of remains is sufficiently large to give grounds for a conjectural reconstruction of the animal; but more frequently the bones are limited to a few caudal vertebræ; and even the larger sets of associated bones come chiefly from the caudal and sacral regions of the vertebral column. With the exception of Macrurosaurus , already described, and also known from a long sequence of large caudal vertebræ, all the remains indicate animals of small or of moderate size, varying between the magnitudes of a sheep and an ox. The majority of the species were characterized by possessing comparatively short tails; though one animal, at least, had a tail in which the vertebræ were more than usually elongated. These remains possess a peculiar interest in being the latest known representatives of the Dinosauria in British geological deposits; and they help to define the limits within which the osteological structure of the