taken place in the past. From the better-preserved specimens that do now and then turn up, we are able to obtain a very exact idea of the construction of the bony cuirass by which Pterichthys and its American cousin were protected, and to make a pretty accurate reconstruction of the entire animal. These primitive fishes had mouths, for eating is a necessity; but these mouths were not associated with true jaws, for the two do not, as might be supposed, necessarily go together. Neither did these animals possess hard backbones, and, while Pterichthys and its relatives had arms or fins, the hard parts of these were not on the inside but on the outside, so that the limb was more like the leg of a crab than the fin of a fish; and this is among the reasons why some naturalists have been led to conclude that vertebrates may have developed from crustaceans. Pteraspis, another of these little armored prevertebrates, had a less complicated covering, and looked very much like a small fish with its fore parts caught in an elongate clam-shell. The fishes that we have so far been considering--orphans of the past they might be termed, as they have no living relatives--were little fellows; but their immediate successors, preserved in the Devonian strata, particularly of North America, were the giants of those days, termed, from their size and presumably fierce appearance, Titantichthys and Dinichthys, and are related to a fish, _Ceratodus_, still living in Australia. We know practically nothing of the external appearance of these fishes, great and fierce though they may have been, with powerful jaws and armored heads, for they had no bony skeleton--as if they devoted their energies to preying upon their neighbors rather than to internal improvements. They attained a length of ten to eighteen feet, with a gape, in the large species called Titanichthys, of four feet, and such a fish might well be capable of devouring anything known to have lived at that early date.