Professor Marsh was able to show that the extremity of the tail in Rhamphorhynchus sometimes expands into a strong terminal caudal membrane of four-sided somewhat rhomboidal shape. He regards this membrane as having been placed vertically. It is supported by delicate processes which represent the neural spines of the vertebræ prolonged upward. They are about fifteen in number. A corresponding series of spines on the lower border, termed chevron bones, equally long, were given off from the junctions of the vertebræ on their under sides, and produced downward. This vertical appendage is of some interest because its expansion is like the tail of a fish. It suggests the possibility of having been used in a similar way to the caudal fin as an organ for locomotion in water, though it is possible that it may have also formed an organ used in flight for steering in the air. [Illustration: FIG. 28. EXTREMITY OF THE TAIL OF _RHAMPHORHYNCHUS PHYLLURUS_ (MARSH) Showing the processes on the upper and under sides of the vertebræ which make the terminal leaf-like expansion] The tail vertebræ from the Cambridge Greensand are mostly found isolated or with not more than four joints in association. They are very like the slender type of neck vertebræ seen in long-necked pterodactyles, but are depressed, and though somewhat wider are not unlike the tail vertebræ of the Rhamphorhynchus. The pneumatic foramen in them is a mere puncture. They have no transverse processes or neural spines, nor indications of ribs, or chevron bones. The hindermost specimens of tail vertebræ observed have the neural arch preserved to the end, as among reptiles; whereas in mammals this arch becomes lost towards the end of the tail. The processes by which the vertebræ are yoked together are small. There is nothing to suggest that the tail was long, except the circumstance that the slender caudal vertebræ are almost as long as the stout cervical vertebræ in the same animal. No small caudal vertebræ have ever been found in the Cambridge Greensand. The tail is very short, according to Professor Williston, in the toothless Ornithostoma in the Chalk of Kansas. CHAPTER X THE HIP-GIRDLE AND HIND LIMB The bones of the hip-girdle form a basin which incloses and protects the abdominal vital organs. It consists on each side of a composite bone, the unnamed bones--_ossa innominata_ of the older anatomists--which are each attached to the sacrum on their inner side, and on the outer side give attachment to the hind limbs. As a rule three bones enter into the borders of this cup, termed the acetabulum, in which the head of the thigh bone, named the Femur, moves with a more or less rotary motion.