Not only was the tail greatly elongated in swimming reptiles, but it was also more or less flattened. In the beginning of water adaptation the flattening was throughout the tail, as in the living alligators and crocodiles. As the adaptation to water life became more perfect, the flattening became more and more restricted to the extremity; that is, the flattening begins like that of a salamander and in the end becomes like that of a fish, a terminal fin. And some of the actual stages in the evolution of the fish-like fin have been observed by Dr. Merriam in the earlier and more primitive ichthyosaurs of California. In those animals swimming chiefly in a horizontal direction the tail fin has become like that of fishes, that is, vertical; but in those animals which use the tail chiefly for ascending and descending rapidly in the water the fin is developed in a horizontal position, examples of which are seen in the flukes of whales and sirenians. All animals living upon the land require firm articulations between the different bones of the skeleton, and especially between the vertebrae, for the support and control of the body. Among aquatic animals there is a strong tendency toward looseness of joints, with increasing flexibility. Fishes have the articular processes between the arches of the vertebrae feebly or not at all developed, and the centra or bodies of the vertebrae have thick pads of cartilage between them. Firm union between the vertebrae would restrict freedom of movement, and firmness is not required when the body is surrounded on all sides by water of nearly the same specific gravity as the body itself. And it is doubtless for the same reasons that the articulations of all strictly aquatic reptiles have for the most part become looser and less firm, especially those between the different vertebrae. The same looseness of articulation is also found in the ribs of aquatic animals. In most animals, and in all those which walk erect, like the mammals, each rib is firmly attached to the backbone by two distinct joints, the head and tubercle, with an interval between them. This double attachment prevents much in-and-out movement of the ribs and gives a firm support for the attachment of the muscles of respiration, as well as for those supporting the viscera. This firmness is unnecessary in animals living always in the water, and the ribs therefore in all aquatic animals tend to become single-headed and loose. The lower or capitular articulation has been lost in part, or almost wholly, in many cetaceans. It has been said that a whale cast up on land will die of suffocation, not for the lack of air, for it is an air-breathing animal like ourselves, but because it can no longer use its respiratory muscles attached to the loosely articulated ribs; it suffocates because the ribs collapse.