The specimen now described was discovered several years since by J. F. Walker, Esq., M.A., F.G.S., among some fossils gathered at a coprolite-washing in the Upper Greensand, from near the railway-bridge at Ditton, N.E. of Cambridge. Mr. Walker recognized the specific importance of his fossil; and, from a cast, I made a brief note, enrolling the species in the Cambridge Greensand fauna as Ichthyosaurus Walkeri . No other bone presumably referable to the same species is known to have been found; and, as with many of the associated fossils, abrasion has done its work upon this femur in a way to suggest that, like many disfigured recent bones to be picked up on our own shores, these Greensand exuviæ were rolled on a pebbly beach before deposition in the bed of phosphatic nodules at the base of the deposit. The bones of the extremities of Ichthyosaurians, as was pointed out by Mr. Hawkins, afford excellent characters by which species may be defined; but in this ordinal group no sufficient description of the skeleton has been made to assist comparison of specimens with a type, perhaps because the varieties of structure in the different genera confounded under the name of Ichthyosaurus are such as to make a comprehensive diagnosis of the several bones a task of difficulty. “ The femur of Ichthyosaurus is a strong short bone, with a small compressed distal end, having its greatest extension at right angles with the greatest width of the head. The distal end shows two