From the middle of the outer convex border of the oval remains of these cups for the cerebral hemispheres, a strong blunt ridge descends obliquely down the sides of the bone to terminate the compressed anterior end of the bone just in front of the hypapophysial ridge of the sphenoid. Above this ridge the bone is much compressed anteriorly, forming a strong straight mesial keel above, which rapidly approximates to the base; the height of the bone in front being one inch and a half, which is also its extreme length. The region below the oblique ridge is a concavity, but it is a little compressed from side to side behind, and has the same anterior compression, so that the elongated oval of the fracture at the anterior end of the bone is only three-eighths of an inch wide. The superior ridge will probably have supported the frontals, and the anterior end would terminate in the orbito-sphenoid. The lateral ridges appear to correspond with what Prof. Huxley has described in the Ostrich as the ridge indicative of a supra-presphenoid ossification pointed out by Kölliker. The groove which is here noticed on the cerebral surface may indicate the same division. If so, the upper and anterior part of the mass would be the ethmoid. This mass offers a considerable resemblance to the frontal portion of the skull of a dolphin (_e. g._ Delphinus delphis) from which the maxillary, premaxillary, palatine and nasal bones have been removed. But in the Porpoise the mesial ridge dividing the cerebral hemispheres is not prolonged so far forward as in the Pterodactyle; the cranial bones are often as smooth on the inside. Notwithstanding Wagner's assurance that the Pterodactyle skull is very like a Monitor's, he would have looked in vain for an ossification in Monitor, Iguana, or other Lizards, comparable with this mass. And although the brain is closed in front by bones in Serpents, it is by the frontal bones, which form a covering for nearly the whole of the conical cerebrum. Nor in the Crocodile is there any ossified mass in front of the brain, although the brain approximates nearer to Birds than is the case with other living Reptiles. Among Birds such a structure as that of the Pterodactyle is characteristic, but no bird has it so massive and mammal-like, though an approximation is made in some thick-skulled birds like _Ciconia marabou_. And in birds it usually is prolonged much further forward than appears to have been the case with Pterodactyle, where from the rapid tapering of the mass in front it appears to have ended in a vertical ridge like that in Parrots and Birds with a moveable beak. In Birds there is usually a median ridge dividing the cerebral hemispheres, but there is also often a small olfactory lobe prolonged in front of the cerebrum, to which nothing analogous is indicated in these fossils. NATURAL MOULD OF THE BRAIN CAVITY OF A CAMBRIDGE ORNITHOSAURIAN[U]. (Cast.) Pl. 11.