It has been only within a few years that we have had any knowledge whatever of them, and that knowledge is still very incomplete, too incomplete to justify any attempt to picture them as living animals, even though we take the liberties that some of our illustrators of extinct animals feel warranted in assuming. The first known specimens of these “sea-reptiles”—for that is the meaning of the word Thalattosauria—were discovered and described by Professor J. C. Merriam less than ten years ago, and all our knowledge of these animals is due to the same author, who has studied attentively the known specimens, all of which are preserved in the museum of the University of California. The first discovered fragmentary specimens were confounded with those of early ichthyosaurs, from the Upper Triassic rocks of northern California with which they were associated. No specimen has yet been found that is even approximately complete; some parts of the skeleton are not yet known, even from fragmentary remains, and not till other and more complete specimens have been found will it be possible to determine the real form of the living animals or to decide what their nearest relationships with other reptiles were. Professor Merriam thinks that they were related most closely with the Rhynchocephalia (p. 176) of which the _Sphenodon_, or tuatera, of New Zealand is the only living representative, but whose direct genealogical history runs back nearly or quite to the time in which the thalattosaurs lived. On the other hand, there are so many resemblances to the mosasaurs shown in the remains that have been discovered, that it is possible the thalattosaurs were only a short-lived branch of the primitive lizards, which we also know were in existence at the time when the thalattosaurs lived. However, even though they resembled the mosasaurs, there could have been no direct genealogical relationships between them, for it is quite certain that the thalattosaurs very soon went out of existence, leaving no descendants. But it matters little which were the land forbears of the thalattosaurs; they present such distinct adaptations to water life—characters all their own—that their ancestral kinship may well be left to the future researches of the curious paleontologist. For the present, at least, they may well be placed in an order of reptiles all their own, as Professor Merriam has proposed—the Thalattosauria. [Illustration: FIG. 82.—Skull of _Thalattosaurus_. (After Merriam)]