some animals that were intermediate in structure between them, and which made their appearance at some time not later than the Silurian period. The ostracoderms were the only ones known to science in anyway likely to fulfil these requirements; it was, therefore, of the utmost importance to learn something more about these mysterious, extinct animals, for they appeared to contain the solution of the whole problem.
The first opportunity to test this side of the problem came in 1900-01, with a half-year's leave of absence from college duties. The prospects of success, however, were very small, especially for one trained as a laboratory morphologist and embryologist, and without experience in field geology or in paleontology. There was a bare chance that a reexamination from another point of view of the material preserved in the British Museum and other institutions, and that had already been studied by Huxley, Lankester, Traquair, Woodward and others, might reveal some suggestive details overlooked by these past masters of the subject. Failing that, it would be necessary to go into the field and dig up new material that, to serve our purpose, would have to be more perfectly preserved than any that had, by chance, been found in the preceding three quarters of a century. It was not a promising outlook, but the opportunity was gladly accepted.
We first visited the great museums of England and Scotland, and the localities where, in the early thirties, Hugh Miller unearthed the first specimens of these animals known to science, which he afterward described with such remarkable literary skill and enthusiasm in his "Footprints of the Creator," and in the "Old Bed Sandstone."
But the best material available in England and Scotland was provokingly incomplete in regard to the very structures it was most important for us to know about. However, some unexpected and suggestive details were found that greatly added to the already keen excitement of the search (Fig. 4).
It was then decided to visit the famous Silurian quarries on the Island of Oesel, in the Baltic Sea, and the museums of St. Petersburg and Moscow, where many of the Oesel fossils were preserved.
The representatives of the ostracoderms (Tremataspis) found in the Silurian rocks of the Island of Oesel are only about three inches long. But in spite of their small size, they are, in some respects, admirably preserved in a soft, fine-grained limestone; and they promised to yield important data. In fact, the specimens that were obtained there showed the presence of jointed appendages and shell-covered, stalk-like eyes. These structures were unlike those of true fishes and more like what one would expect to find in some free-swimming seascorpion. In that respect the results were highly satisfactory, and added still more evidence in favor of our first supposition (Fig. 5).
