Although their remains are among the most abundant and widely distributed of all fossil reptiles, the plesiosaurs as a whole are less perfectly known than either the ichthyosaurs or the mosasaurs, and it has been within a comparatively few years only that an approximately complete knowledge of any form has been obtained. This is partly due to the fact that the order comprises vastly more kinds, more species, genera, and families than does any other order of marine reptiles; partly because their remains, though widely distributed over the earth, and in rocks of many geological epochs, are seldom found completely preserved; usually specimens comprise only a few bones or single bones, and complete skeletons are rare. Were there but few kinds, the many specimens discovered would mutually supplement each other, finally completing our knowledge; but the fragments of many kinds only add to our confusion. Nevertheless, because the plesiosaurs lived so long in geological history, their remains are found in rocks of many different kinds, and since it is improbable that any of them had great specific longevity, it is very probable that all these described species, or most of them, often made known from single bones, will eventually be found to be distinct, and that many more will be added to them. It does not seem improbable that within the next forty or fifty years not less than a hundred species of plesiosaurs will have been discovered in North America alone. At the present time perhaps that many have been described from the whole world. When Blaineville gave the name Plesiosauria to the aquatic reptiles described by Conybeare, Cuvier, and others, he had no knowledge of others of an intermediate kind between them and land reptiles. His group-term then can be properly applied only to the truly aquatic forms, and Owen’s name Sauropterygia becomes available in a wider sense to include all the known types belonging to the order of which the plesiosaurs form a part. Of this order then there are two clearly marked divisions or suborders, the Plesiosauria and the Nothosauria, the former having a complete aquatic adaptation, the latter only a partial one. While the two suborders are evidently allied, some authors have suggested that their differences are only familial; others have thought that they are really orders. We shall see how close the relationships are. PLESIOSAURIA It was Dean Buckland who facetiously likened the plesiosaurs to a snake threaded through the shell of a turtle, and the simile was not an inapt one in his day. The vernacular designation of them—long-necked lizards—conveys the same impression of their chief peculiarity, but the name is less applicable than it once was, since recent discoveries have brought to light forms with a relatively short neck.