The small piece of premaxillary on which this species is founded indicates a small animal, and nearly resembles the jaw of _O. microdon_. It is scarcely an inch long; nearly 9/16ths inch high behind, and nearly 7/16ths of an inch high in front, so that it tapers very rapidly, and could scarcely have been an inch longer in front. The nose is well rounded, but the sides are a little concave, and become well pinched in in the middle, behind, showing the near approach as I think to the nostril. The palate half an inch broad, is divided into two concave channels by the strong and sharp median ridge, which projects below the alveolar margins. The dental margins are not rounded as in _C. microdon_, but flattened, making more than a right angle with both the outer side-wall and palate. The interspaces between the teeth are rough, looking as though they had supported minute teeth. The alveolar margin is a tenth of an inch wide; along it are the perfectly circular sockets, a sixteenth of an inch in diameter. There are 3 sockets between 5/8 of an inch, so that they are separated by 3 times their diameter. The palate is obliquely impressed with blood-vessels running forward to the teeth from the median ridge. The points in which this jaw differs from that of _O. microdon_ are that in this species the teeth are circular instead of being oval; that the interspaces here are as long as in that species, though this jaw is only two-thirds the width; that instead of having a sharp keel on the upper surface, this has a well rounded roof. That though the jaw is scarcely higher than it is wide, it shows strong furrows running up to the nares, while in _O. microdon_, though the proportions are the same, the sides are perfectly flat without trace of pinching in, while the line of the nasal opening is indicated by a faint furrow running all along the jaw. And lastly it differs in size, which, where the sutures are lost, may be important in discriminating forms. VI. Ornithocheirus microdon (Seeley). Case. Comp. Tablet. Specimen. =J= _c_ 29 1-2 Premaxillary bone. The fossil is nearly 1-3/4ths inch long, and at the proximal end, where it is less than 3/4ths of an inch high, has flat sides, which converge to form a keel which is depressed anteriorly and rounded so that where fractured in front the bone is 7/16ths of an inch deep. The palatal surface contains two wide concave channels, between which descends a sharp median ridge, which behind becomes more prominent than the alveolar border. The palate is 5/8ths of an inch wide. The alveolar margins are compressed and rounded. The small tooth-sockets are oval, and four are contained in 1-1/8th inch; they look downward.