The appearance of these sea-scorpions is given in Figs. 7 and 8, representing Stylonurus, Slimonia, Pterygotus, Eurypterus. They must have been in those days the tyrants of the deep, for specimens of Pterygotus have been found over six feet in length. At this time, then, by every criterion hitherto used, by the multitude of species, by the size of individual species, which at this period reached the maximum, by their subsequent decay and final extinction, we must conclude that these forms were in their zenith, that the predominant race at this time was to be found in this group of arthropods. Just previously, the sea swarmed with trilobites, and right into the period when the Gigantostraca flourished, the trilobites {26}are still found of countless forms, of great difference in size. The whole period may be spoken of as the great trilobite age, just as the Tertiary times form the mammalian age, the Mesozoic times the reptilian age, etc. From the trilobites the Gigantostraca and Xiphosura arose, as evidenced by the embryology of Limulus, and, therefore, in the term trilobite age would be included the whole of those peculiar forms which are classified by the names Trilobita, Gigantostraca, Xiphosura, etc. Of all these the only member alive at the present time is Limulus, or the King-Crab. [Illustration: FIG. 6.--A TRILOBITE (_Dalmanites_) (after PICTET). Dorsal view.] [Illustration: FIG. 7.--_Eurypterus remipes_ (after NIESKOWSKI). Dorsal view.] As, however, the term 'trilobite' does not include the members of the king-crab or sea-scorpion groups, it is advisable to use some other term to represent the whole group. They cannot be called crustaceans or arachnids, for in all probability they gave origin to both; the nearest approach to the Trilobite stage of development at the present time is to be found perhaps in Branchipus (Fig. 10) and Apus (Fig. 9), just as the nearest approach to the Eurypterid {27}form is Limulus. Crustaceans such as crabs and lobsters are of much later origin, and do not occur in any quantity until the late Mesozoic period. The earliest found, a kind of prawn, occurs in the Carboniferous age. [Illustration: FIG. 8.--A, _Pterygotus Osiliensis_ (from SCHMIDT). B, _Stylonurus Logani_ (from WOODWARD). C, _Slimonia acuminata_ (from WOODWARD).] Korschelt and Heider have accordingly suggested the name _Palæostraca_ for this whole group, and _Protostraca_ for the still earlier {28}arthropod-like animals which gave origin to the trilobites themselves. This name I shall adopt, and speak, therefore, of the _Palæostraca_ as the dominant race at the time when vertebrates first appeared.