One of the most peculiar and distinct of those unique types alluded to above is that of the Rudistes, a singular Bivalve, in which the lower valve is very deep and conical, while the upper valve sets into to it as into a cup. The subjoined woodcut represents such a Bivalve. These Rudistes are found suddenly in the Urgonian deposits; there are none in the two preceding sets of beds; they disappear in the three following periods, and reappear again in great numbers in the Cenomanian, Turonian, and Senonian periods, and disappear again in the succeeding one. These can hardly be missed from any negligence or oversight in the examination of these deposits, for they are by no means rare. They are found always in great numbers, occupying crowded beds, like Oysters in the present time. So numerous are they, where they occur at all, that the deposits containing them are called by many naturalists the first, second, third, and fourth _bank_ of Rudistes. Which of the ordinary Bivalves, then, gave rise to this very remarkable form in the class, allowed it to die out, and revived it again at various intervals? This is by no means the only instance of the same kind. There are a number of types making their appearance suddenly, lasting during one period or during a succession of periods, and then disappearing forever, while others, like the Rudistes, come in, vanish, and reappear at a later time. [Illustration: Rudistes.] I am well aware that the advocates of the development-theory do not state their views as I have here presented them. On the contrary, they protest against any idea of sudden, violent, abrupt changes, and maintain that by slow and imperceptible modifications during immense periods of time these new types have been introduced without involving any infringement of the ordinary processes of development; and they account for the entire absence of corroborative facts in the past history of animals by what they call the "imperfection of the geological record." Now, while I admit that our knowledge of geology is still very incomplete, I assert that just where the direct sequence of geological deposits is needed for this evidence, we have it. The Jurassic beds, without a single modern scaly Fish, are in immediate contact with the Cretaceous beds, in which the Fishes of that kind are proportionately almost as numerous as they are now; and between these two sets of deposits there is not a trace of any transition or intermediate form to unite the reptilian Fishes of the Jurassic with the common Fishes of the Cretaceous times. Again, the Cretaceous beds in which the crowded banks of Rudistes, so singular and unique in form, first make their appearance, follow immediately upon those in which all the Bivalves are of an entirely different character. In short, the deposits of this year along any sea-coast or at the mouth of any of our rivers do not follow more directly upon those of last year than do these successive sets of