Forty-three specimens are mounted to exemplify the variations in size and characters of cervical vertebræ. These for the most part are specific characters; and between the axis and the first dorsal vertebra the variations in an individual were slight. [Those nearest to the back, as in birds, are widest in front, and have the highest neural arches.] The associated series show commonly four cervical vertebræ behind the axis, and in two cases apparently five; never more. So that as seven appears to be the number of true cervical vertebræ in most if not all of the German Pterodactyles, it may be presumed that the Cretaceous Ornithosaurians also had this character in common, with Mammals, and probably as persistent. In Iguana there are 6, in Monitor 7, and in Crocodile 8. The centrum is united to the neural arch as in birds, without a trace of suture; sometimes the neural arch is no wider than the centrum, sometimes it extends over the centrum on each side. Those forms with a narrow neural arch have the neural spine high, and its sides look forward as well as outward. The pneumatic foramen is oblique. An example is figured by Prof. Owen, in the memoir on Pterodactylus simus, pl. 2, fig. 4. The forms with a wide neural arch have the neural spine rising from the middle of the dorsal surface, erect and equally compressed from side to side. The pneumatic foramen is horizontal. An example is figured in Prof. Owen's memoir on Pt. simus, pl. 2, fig. 1. These two forms of cervical vertebræ may be regarded as typifying two genera. In both forms many characters occur in common, and as the specimens illustrative of special modifications will be described hereafter, the following description has been made to embrace the chief characteristics of these vertebræ in Cretaceous Ornithosaurians. The inferior aspect of the centrum is oblong (being narrower than long), or quadrate; when quadrate the additional lateral expansions are external to the pneumatic foramina, and are formed by the neural arch and zygapophyses. The centrum proper is a little wider in front than behind, and the side outlines are concave. The base of the centrum is flattened, or more or less hollow, or more or less tumid and regularly convex; in front there is often a mesial ridge, which never reaches the posterior articulation, and forms a prominent tubercle at the base of the anterior articulation. At the posterior end the outline of the centrum is concave, and mesially the bone has a hollow corresponding to the tubercle in front of the adjacent vertebra; and the part of the centrum on each side is prolonged slightly into a strong rounded or flattened tubercle below the side borders of the posterior articulation; these posterior processes, in vertebræ in situ, fitted, on each side of the mesial anterior process of the vertebra behind, on to concavities more or less marked. Analagous processes are developed in the cervical vertebræ of many birds.