There is very much doubt, very much uncertainty, among paleontologists about the classification of reptiles. No two writers agree on the number of orders, or the rank of many forms. Some recognize twenty or more orders, others but eight or nine. And this doubt and uncertainty are due chiefly to the many discoveries of early forms that have been made during the past twenty years. The many strange and unclassifiable types which have come to light in North America, South Africa, and Europe have thrown doubt on all previous classificatory schemes, have weakened our faith in all attempts to trace out the genealogies of the reptilian orders; and classification is merely genealogy. It is only the paleontologist who is competent to express opinions concerning the larger principles of classification of organisms, and especially of the classification of reptiles. The neozoölogist, ignorant of extinct forms, can only hazard guesses and conjectures as to the relationships of the larger groups, for he has only the specialized or decadent remnants of past faunas upon which to base his opinions. About some things we can be quite confident; about some groups opinions have crystallized, and we all agree, except perhaps on trifles. The dinosaurs, the pterodactyls, the crocodiles, for instance, offer only minor problems to perplex the systematist, but the origin and the relations, not only of these, but also of nearly all the others, are still involved in obscurity. The question, whence came the ichthyosaurs, the plesiosaurs, the turtles, etc., seems almost as far from solution as it did fifty years ago. With every problem solved a dozen more intrude themselves upon us. Hence, classification simply represents the present condition of our knowledge, our present opinions as to genealogies. It was the fashion a dozen years ago to draw all sorts of genealogical trees on the slightest pretext, to trace in beautifully clear lines the precise descent of all kinds of animals; and very few have been worth the paper on which they were printed. When facts are numerous enough, conclusions are patent even to the novice; when facts are few and obscure, one can guess about as well as another. In general, it may be said that the older a group of animals is the more abstruse are the problems presented; first, because of the lack of abundant material; second, because the forms speak to us in an unfamiliar language that we cannot easily interpret. The classification of the mammals approaches more nearly the ultimate truth than does that of any other group of organisms, because we know more about the extinct forms than we do of any other class, and also because we know more about the living forms than we do about any other living animals.