since some are so nearly alike that their relationships or possible identity would probably be established after long search. This will serve to make clear how very difficult it is, for the most part, to correlate exactly the geological formations in remote regions of the earth, or even sometimes in adjacent regions where the fossils are scanty, or the conditions under which the animals had lived were very different. There are long periods of time, millions of years at a stretch perhaps, throughout which our knowledge amounts to little or nothing concerning many land reptiles which we are sure must have existed abundantly. No better example of our oftentimes scanty knowledge can be cited than the following. Until within the past fifteen years it was thought that true land lizards, of which there are about eighteen hundred species now living, dated back in their history no farther than about the close of the great Secondary Period, or the Age of Reptiles. But a single skull of a true land lizard has been discovered in the Triassic deposits of South Africa, a skull of a form so nearly like that of the modern iguana of America that its discoverer, Dr. Broom, has called it _Paliguana_. The lizards must have been in existence, probably many thousand species of them, during all the great interval of time between the Middle Triassic and the close of the Cretaceous, since it is a law which can have no exception, that a type of life once extinct never reappears. The “ancient iguanas” of the Trias must have been the forbears of many, if not all, of the lizards of later times, though nothing is known of their descendants through a period of time which can be measured only by millions of years. However, notwithstanding these imperfections of our geological records, we know very much more about extinct reptiles than we do about living ones, so far at least as those parts capable of preservation in the rocks are concerned. Were our knowledge of reptiles confined to the forms now living upon the earth it would be relatively very incomplete since, aside from the lizards and snakes, they are merely the remnants of what was once a mighty class of vertebrates.