dorsal ribs of the claw-like process, which extends laterally from rib to rib as a marked feature in many birds. Its presence or absence may not be important, because it is represented by fibro-cartilage in the ribs of crocodiles, and may be a small cartilage near the head of the rib in serpents, and is only ossified in some ribs of the New Zealand reptile Hatteria. So that it might have been present in a fossil animal without being ossified and preserved. Although the structure is associated with birds, it is possibly also represented by the great bony plates which cover the ribs in Chelonians, and combine to form the shield which covers the turtle's back. The structure is as characteristic of reptiles as of birds, but is not necessarily associated with either. [Illustration: FIG. 26 The upper figures show the side and back of a dorsal vertebra of Ornithocheirus compared with corresponding views of the side and back of a dorsal vertebra of a Crocodile] There are two remarkable modifications of the early dorsal vertebræ in some of the Cretaceous Pterodactyles. First, in the genus Ornithodesmus from the Weald the early dorsal vertebræ are blended together into a continuous mass, like that which is found in the corresponding region of the living Frigate-bird, only more consolidated, and similar to that consolidated structure found behind the dorsal vertebræ, known as the sacrum, made by the blending of the vertebræ into a solid mass which supports the hip bones. Secondly, in some of the Cretaceous genera of Pterodactyles of Europe and America the vertebræ in the front part of the back are similarly blended, but their union is less complete; and in genera Ornithocheirus and Ornithostoma--the former chiefly English, the latter chiefly American--the sides of the neural spines are flattened to form an oval articular surface on each side, which gives attachment to the flattened ends of their shoulder-blade bones named the scapulæ. This condition is found in no other animals. Three vertebræ appear to have their neural arches thus united together. The structure so formed may be named the notarium to distinguish it from the sacrum. SACRUM