The ossa innominata have been determined as right and left on the supposition that the pelvis of the Dimorphodon is in situ, and from the general correspondence of the form of the constituent elements with elements of the pelvis in the lower mammals, reptiles, and birds. Each os innominatum shows a hemispherical acetabulum which is slightly elongated in antero-posterior extent In the Dimorphodon the bone which is superior to the cup, that is to say, which extends dorsally along the sacral vertebræ is prolonged anteriorly as a strong narrow straight style, the base of which is seen in the parts marked _Ilium_ in Nos. 1 and 6. A more perfect example may be studied in a pelvis from the Cambridge Greensand preserved in the collection of the Geological survey. Posterior to the acetabulum a similar but stronger bony style extends for more than the length of the acetabulum, curving slightly downward at its posterior part. The dorsal outline of this portion of the bone is slightly concave. The posterior horn like the anterior horn forms part of the ilium which constitutes the upper half of the acetabular cup. The os innominatum contracts in antero-posterior extent below the acetabulum, and immediately widens again in a thin concave bony expansion. The anterior or pubic outline is comparatively straight, and at right angles with the ilium; the posterior or ischiac outline is deeply cupped where the ischium unites with the ilium, and becoming straight extends backward at a considerable angle. The ischium contributes less to the pelvic cup than either the ilium or pubis; it is flat in front and convex on the visceral side, rounding into the narrow flattened posterior side. The pubis is separated from the ischium by a suture extending vertically through the obturator foramen. The obturator foramen [seen in No. 9] is small and oval, less than half the diameter of the acetabulum, situated below its ventral border. It passes obliquely downward and a little forward, and its opening makes the exterior aspect of the pubis concave; the visceral aspect of the pubis is convex from side to side like the ischium. The sacral aspect of so much of the os innominatum as is seen, is concave from the dorsal to the ventral margins, and is cupped behind and below the acetabulum, the surface being rough. Among reptiles the ilium is chiefly behind the acetabulum, in mammals it is chiefly in front. In the over-lapping group, Aves, it extends both ways. Among the Amphibia the ilium is chiefly anterior to the acetabulum. In Crocodiles it has a slight extension both ways, in Dinosaurs the extensions are more marked and the whole arrangement approximates to birds. But among animals which have been affiliated with reptiles the Dicynodonts are the only order in which there is a pelvis so mammalian and massive. If the ilium of the Monotreme genus Echidna had a posterior extension, the pelvis would be altogether comparable with the pelvis of this Pterodactyle, and