Table of Periods in the Cretaceous Epoch.
| Maestrichtian | Chalk. | |
| Senonian | ||
| Turonian | Chalk Marl. | |
| Cenomanian | ||
| Albian | Green Sands. | |
| Aptian | ||
| Rhodanian | ||
| Urgonian | Wealden. | |
| Neocomian | ||
| Valengian | ||
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.
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 beds of past ages follow upon each other. In making these statements, I do not forget the immense length of the geological periods; on the contrary, I fully accede to it, and believe that it is more likely to have been underrated than overstated. But let it be increased a thousand-fold, the fact remains, that these new types occur commonly at the dividing line where one period joins the next, just on the margin of both.
For years I have collected daily among some of these deposits, and I know the Sea-Urchins, Corals, Fishes, Crustacea, and Shells of those old shores as well as I know those of Nahant Beach, and there is nothing more striking to a naturalist than the sudden, abrupt changes of species in passing from one to another. In the second set of Cretaceous beds, the Neocomian, there is found a little Terebratula (a small Bivalve Shell) in immense quantities: they may actually be collected by the bushel. Pass to the Urgonian beds, resting directly upon the Neocomian, and there is not one to be found, and an entirely new species comes in. There is a peculiar Spatangus (Sea-Urchin) found throughout the whole series of beds in which this Terebratula occurs. At the same moment that you miss the Shell, the Sea-Urchin disappears also, and another takes its place. Now, admitting for a moment that the later can have grown out of the earlier forms, I maintain, that, if this be so, the change is immediate, sudden, without any gradual transitions, and is, therefore, wholly inconsistent with all our known physiological laws, as well as with the transmutation-theory.
There is a very singular group of Ammonites in the Cretaceous epoch, which, were it not for the suddenness of its appearance, might seem rather to favor the development-theory, from its great variety of closely allied forms. We have traced the Chambered Shells from the straight, simple ones of the earliest epochs up to the intricate and closely coiled forms of the Jurassic epoch. In the so-called Portland stone, belonging to the upper set of Jurassic beds, there is only one type of Ammonite; but in the Cretaceous beds, immediately above it, there set in a number of different genera and distinct species, including the most fantastic and seemingly abnormal forms. It is as if the close coil by which these shells had been characterized during the Middle Age had been suddenly broken up and decomposed into an endless variety of outlines. Some of these new types still retain the coil, but the whorls are much less compact than before, as in the Crioceras; in others, the direction of the coil is so changed as to make a spiral, as in the Turrilites; or the shell starts with a coil, then proceeds in a straight line, and changes to a curve again at the other extremity, as in the Ancyloceras, or in the Scaphites, in which the first coil is somewhat closer than in the Ancyloceras; or the tendency to a coil is reduced to a single curve, so as to give the shell the outline of a horn, as in the Toxoceras; or the coil is entirely lost, and the shell reduced to its primitive straight form, as in the Baculites, which, except for their undulating partitions, might be mistaken for the Orthoceratites of the Silurian and Devonian epochs. I have presented here but a few species of these extraordinary Cretaceous Ammonites, and, strange to say, with this breaking-up of the type into a number of fantastic and often contorted shapes, it disappears. It is singular that forms so unusual and so contrary to the previous regularity of this group should accompany its last stage of existence, and seem to shadow forth by their strange contortions the final dissolution of their type. When I look upon a collection of these old shells, I can never divest myself of an impression that the contortions of a death-struggle have been made the pattern of living types, and with that the whole group has ended.
| Crioceras. | Turrilites. | Ancyloceras. | Scaphites. |
| Toxoceras. | Baculites. |