I Presume that most English students of Mesozoic Reptiles are acquainted, at least by report, with the magnificent collection of the remains of Sauropterygians and other Saurians from the Oxford Clay of Northamptonshire in the possession of Mr. A. N. Leeds, of Eyebury, near Peterborough. Those, however, who have not had the good fortune to see this unrivalled collection, can have no idea of its richness, or of the light it throws on the organization and affinities of the Sauropterygians of the later Jurassic seas.