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. [Illustration: Crioceras.] [Illustration: Turrilites.] [Illustration: Ancyloceras.] [Illustration: Scaphites.] [Illustration: Toxoceras.] [Illustration: Baculites.]