following periods, and often in the very same deposits in which the first vertebrates are found. Moreover, during the Silurian and Devonian Fig. 1. Trilobite (Mesonacis vermontana Hall). Lower Cambrian. One of the marine Arachnids. periods, and living in intimate association with the declining marine arachnids and the earliest vertebrates, there was known to exist a peculiar class of animals called the Ostracoderms. Very little was known about them, for their remains were fragmentary and their meaning doubtful. Some of the species were regarded as vertebrates, others as invertebrates, and some of them showed a superficial resemblance to the sea-scorpions, as had been noticed by the older anatomists (Fig. 2). But after considerable discussion and a thorough reexamination of the available material, mainly by Huxley and Lankester, it was definitely decided that they were fishes, probably a very specialized kind; of no great importance morphologically. They were then forgotten, or at any rate their very existence remained quite unknown to many zoologists.

In view of their unfishlike appearance and their great antiquity, it is astonishing that no one suggested they might be very primitive vertebrates, dr perhaps an even more remote ancestral stock, until the author did So in his first paper on "The Origin of Vertebrates."^[1]

This oversight was largely due to the fact that, at that time, zoologists firmly believed that the most primitive vertebrates were like sharks, or like Amphioxus; that is, animals that had little or no skeleton, while the Ostracoderms were, encased in an extraordinary dermal armor, in some respects quite like that of a trilobite or sea-scorpion. Something appeared to be wrong, either in the facts or in the conclusions. Could it be possible that, after all, the ostracoderms were not true fishes, but a new class of animals, a class intermediate between the fishes and the sea-scorpions? In some respects they looked as much like the one as the other, and they appeared at the right time historically to be the long-sought-for missing links between vertebrates and invertebrates. If they really were transition forms, that would fully account for the resemblance between the modern arachnids and