The only portion of the specimen now to be described is the large region at each side looking downward, which extends from the occipital ridge to the sphenoid. It is an irregular pentangular hollow with many cavities, the hinder of which are for the ear. Two cavities above these, under the widest part of the skull, appear to be a double articulation for the quadrate bone. The outer transverse one with the squamosal is separated by a deep groove from the inner and more vertical one, which may therefore be regarded as with the petrosal bone. These excavations form the posterior half of the pentagon. The anterior half is a smooth rhombus not separable from the basi-sphenoid. Such is the external appearance of the occipital and parietal segments of the skull of a Cambridge Pterodactyle. Each segment forms a large ring of thin bone, inclosing part of a brain-cavity as large as that of a bird and shaped like that of a bird; and which moreover is made up of the same bones as the cranium of a bird; and these are in almost exactly the same proportions as those of the Common Cock. My own investigations do not substantiate Wagner's discovery, that the back part of the skull resembles that of the Monitor. Iguana would have offered a slightly nearer comparison, but they both differ from Cambridge specimens of Pterodactyles in characters like these. In the lizard, The cranial bones do not enclose the brain. There is no division of the back of the skull into an occipital segment and a parietal segment by a girdling crest. The squamosal bone does not enter into the cranial wall. The quadrate bone does not articulate with the wall of the brain-case. While the peculiar backward development of wings of the parietal in a diverging V form, give the Lizard skull an aspect of its own. So that it must be asserted that the differences of these Pterodactyles from Lizards are so wide as to preclude comparison. With the Crocodile, in which the cranial bones are massive, and the quadrate bone firmly packed in the skull, comparison would be no less difficult. The Delphinidæ, in both the form of the jaws and of the back of the head, give some support to Wagler's fancy, in putting the Pterodactyle into his curious creation, the Gryphi[T]. But in the porpoises the parietal bones form as narrow a band as they do in the Duck; and are quite unlike the bones here described. In the Dolphin the two condyles almost unite into one semicircular condyle (in young specimens), owing to the enormous development of the ex-occipitals, which almost if not entirely exclude the basi-occipital from the foramen magnum. The dolphin moreover has no quadrate bone. But notwithstanding the absence of a division into occipital and parietal segments, the form and arrangement of the bones in the skull of the porpoises approximate more to the Cambridge Pterodactyles than is the case with Lizards.