��THE POPULAR SCIENCE MONTHLY
��following periods, and often in the very same deposits in which the first vertebrates are found. Moreover, during the Silurian and Devonian
periods, and living in intimate associa- tion with the declining marine arach- nids 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 verte- brates, others as invertebrates, and some of them showed a superficial resemblance to the sea-scorpions, as had been noticed by the older anato- mists (Fig. 2). But after consider- able discussion and a thorough reexam- ination 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 'co'cjuHe 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 verte- brates, dr perhaps an even more remote ancestral stock, until the author did So in his first paper on " The Origin of Vertebrates." 2
This oversight was largely due to the fact that, at that time, zool- ogists firmly believed that the most primitive vertebrates were like sharks, or like Amphioxus; that is, animals that had little or no skele- ton, 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 con- clusions. 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 his- torically 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
��Fig. 1. Trilobite (Mcsonacis ver- montana Hall). Lower Cambrian, ■g — -^..,Qne of the marine Arachnids.
��Q. J. Mic. Sc, Vol. 31, 1890.