In _front_ is seen a section of the brain-cavity. It is very like in form to the two halves of a pear put together side by side with the stalk downward. I have removed some of the phosphate of lime from the brain-cavity, and although it has not been excavated to the cerebellum, the great depth of the brain is well seen, and the convex character of the cerebral lobes, between which a crest of bone descends mesially as in the ethmo-sphenoid mass next described. At each of the lower outer angles of the brain, extending into the cancellous brain-walls to the outermost film, is an ovoid convexity, covered with a thin film of bone. They entirely correspond with the optic lobes, being in exactly the same position as in birds, only relatively rather small. Underneath the optic lobe on the outside is a small concavity, apparently the articulation for the quadrate bone. The basi-sphenoid mass below the brain is of considerable height, the upper half flat and smooth, the lower half fractured and cancellous. In the main this skull is like the other one, differing chiefly in the depth of the sphenoid, in the mesial ridge between the cerebral lobes, in showing the optic lobes, and in having anchylosed basi-temporal bones. There would hence appear to have been considerable variations in the skulls of Pterodactyles even in the Cambridge Greensand. Orbito-ethmo-sphenoid bone. Pl. 11. Case. Comp. Tablet. =J= _c_ 9 The symmetrical bone which I have so named is a wedge-like mass tapering in front, keeled above; flattened below, and cupped behind on each side. It belonged to a very much larger animal than the last fossil, and probably to a very different genus. The _inferior surface_ is triangular, an inch and an eighth wide behind, at the base, and an inch and a quarter long; but it is broken at both ends. In its longitudinal median line is a strong keel stopping short in front, dying away behind, and forming with the compressed margins a considerable hollow on each side, at the back part of which is a large oval foramen. This surface, though five times the size, corresponds in form, ridges, and foramina with the anterior part of the sphenoid described in the article on the back of the cranium. The _posterior surface_ is at right angles to the inferior one, but its lower third shows only fractured phosphate of lime filling perhaps the anterior part of the pituitary fossa. Its upper part also is broken. But on each side is a large concavity measuring in the fractured fossil an inch and a quarter high, three quarters of an inch wide, and half an inch deep from the unbroken median ridge where the cups become confluent at their base. The whole specimen is two and a quarter inches high. From the determination of the under side it follows that these smooth hollows, over each of which an impressed mesial line descends obliquely outward, are a part of the anterior boundary of the brain.