In all the earliest reptiles, as we have seen, the neck was short, like that of their immediate progenitors, the ancient amphibians. The shoulders were close to the skull, with not more than two vertebrae that could be called cervical. It happens that most of the earliest reptiles, as we know them, were more or less amphibious in habit, and all of them were probably good swimmers; nevertheless in all likelihood reptiles began their career as a class with a very short neck. The earliest known distinctly terrestrial reptiles had a moderately long neck composed of six or seven cervical vertebrae. It may therefore be assumed with much probability that all later reptiles with a greater or less number of cervical vertebrae are specialized animals, so far as the neck is concerned. Most living reptiles have eight cervical vertebrae; a few have nine, and still fewer have but five. Birds may have as many as twenty-four, while all mammals, with two or three exceptions, have the primitive number seven. Among extinct reptiles, however, there were not a few with more numerous neck vertebrae, some having the enormous number of seventy-six. An ordinary fish has apparently no neck whatever, the trunk being seemingly attached to the head, nearly as in the primitive amphibians and primitive reptiles. It is evident that a movable neck of considerable length would not only be of no use to the swiftly swimming fish, but a positive disadvantage to it. The body is quickly and easily turned by the powerful tail fin, and a long neck could be of no use that the tail would not better subserve. It is therefore of interest to learn that, as a rule, aquatic animals of all kinds having a powerful propelling tail have also a short neck, acquired either by the loss of neck vertebrae, or, as in the mammals, by the shortening and coalescence of the normal number of seven. There are very few exceptions to this rule of a short neck and a long tail. Those strange little reptiles of Paleozoic times, the first that we know that returned to the water, the Proganosauria, have not only a long, flattened tail, but also an unduly elongated neck of from nine to twelve vertebrae.