These Devonian corals, like those of the Silurian, belong to the great extinct groups of Tabulate and Rugose corals; groups which present, on the one hand, points of resemblance to the ordinary coral animals of the modern seas, and, on the other, to those somewhat exceptional corals, the Millepores, which are produced by another kind of polyp, the Hydroids. Some of them obviously combine properties belonging to both, as, for example, the radiating partitions with the arrangement of the parts in multiples of four, the horizontal floors, and the external solid wall; and this fact countenances the conclusion that in these old corals we have a group of high and complex organization, combining properties now divided between two great groups of animals, neither of them probably, either in their stony skeletons or the soft parts of the animal, of as high organization as their Paleozoic predecessors. This sort of disintegration of composite types, or dissolution of old partnerships, seems to have been no unusual occurrence in the history of life.[L] [L] Verril has suggested that the Tabulata may be divided into two groups, one referable to Actinoids, the other to Hydroids. If the Devonian witnessed the culmination of the Palæozoic corals, its later stages saw the final decadence of the great dynasty of the Trilobites. Of these creatures there are in the Devonian some large and ornate species, remarkable for their spines and tubercles; as if in this, the latter day of their dominion, they had fallen into habits of luxurious decoration unknown to their sterner predecessors, and at the same time had found it necessary to surround their now disputed privileges with new safeguards of defensive armour. Not improbably the decadence of the Trilobites may have been connected with the introduction of the numerous and formidable fishes of the period.