I am indebted to my friend Mr. A. N. Leeds, of Eyebury, near Peterborough, for the opportunity of bringing under the notice of the Society a very interesting, although unfortunately imperfect, Dinosaurian jaw, recently obtained from the brick-pits in the Oxford Clay near the town named. The specimen comprises the anterior and posterior extremities of the left ramus of the mandible, showing the alveoli of the teeth and the cavity for the articulation of the quadrate. The fractured surfaces are fresh, and it is thus evident that the present imperfect condition of the specimen is due to a blow from the pick of the workman by whom it was disinterred. When entire, its total length was probably about 1 foot. The anterior fragment (Pl. XI. figs. 1, 1 a )comprises the greater portion of the dentary bone, with the symphysis entire; while the hinder moiety ( ibid. figs. 2, 2 a ) includes the articular, and portions of the angular and surangular elements. The dentary bone is somewhat roughened and pitted on its external surface, with a broad symphysial channel; while the symphysis itself is oblique, and in life was evidently united by ligament. Superiorly the outer surface is concave from above downwards, while below the concavity it is traversed by a prominent longitudinal ridge, dividing the proper lateral from the inferior aspect. The alveolar margin is characterized by its abrupt deflection near the middle of its length: the deflected portion falling away continuously to the extremity of the shallow symphysis. The whole