surfaces differ in no respect from those of cervical vertebræ. The inferior surface of the centrum is separated from the sides by two ridges parallel to the lateral concave outlines of the neural arch; they extend from sides of the front, more or less well marked, to the tubercular processes at the base of the sides of the centrum behind. The dice-box shaped area of the centrum so inscribed is usually concave from front to back, and concave from side to side behind, and convex from side to side in the middle; this convexity is only broken in front by the development of the slight mesial hypapophysial ridge. The sides are narrow, flattened, look downward and outward, are a little concave from front to back, round into the centrum and into the neural arch, and show at about the middle a small pneumatic foramen, which is variable in size, but largest in No. 8, and sometimes a mere puncture. The caudal vertebræ differ in many ways from other animals. They have neither transverse processes, neural spines, hypapophyses or hæmapophyses. In the persistence of the neural arch down the tail they resemble reptiles and birds rather than mammals, in which nothing but the centrum persists to the end of the tail. The vertebræ are furnished with vertebral arteries which run through the neural arch parallel to the neural canal, in exactly the same position as do the vertebral arteries in the neck-vertebræ of the Llama. THE BONES OF THE HEAD. Pl. 11, 12. The skull of Dimorphodon differs in form and in many important details of structure from that of Rhamphorhynchus; and both of these types of skull are strikingly unlike that of the short-tailed animals named Pterodactyle. Hence, as it will be shown that the Cretaceous fossils of this class belong to very distinct new genera, there is no reason for assigning to them by anticipation any class of cranial structures. The cranium of this type of animal has never been critically described, and for all that is yet known to the contrary Pterodactyles may differ between themselves as much as birds or mammals. Their affinities have been unknown. Therefore, before describing bones it may be desirable to state the grounds on which the several specimens are referred to the Ornithosauria. The fossils on which this section of the memoir is founded are, the basi-occipital and basi-temporal bones, the anterior portion of a cranium, the back parts of four crania, facial bones, and the quadrate and quadrato-jugal.