The interest of this investigation as applied to Limulus is increased by the fact that this creature is the near ally of Trilobites, Eurypterids and other Crustaceans which were abundant in the earlier geological ages, and whose footprints are probably among the most common we find on the rocks. [Illustration: Rusichnites Grenvillensis, Billings a "Bilobite." Probably the Cast of a Crustacean burrow.] Lastly, on this part of the subject, it is to be observed that many other marine animals, both crustaceans and worms, make impressions resembling in general character those of Limulus. In addition to those already mentioned, Nathorst and Bureau have shown that various kinds of shrimps and lobster-like Crustaceans, when swimming rapidly by successive strokes of the tail, make double furrows with transverse ridges resembling those of Bilobites, and there are even some mollusks which by the undulations of the foot or the hook-like action of its anterior part, can make similar trails. A question arises here as to the value of such things as fossils. This depends on the fact that many creatures have left their marks on the rocks when still soft on the sea bottom, of which we have no other indications, and it also depends on our ability to understand the import of these unconscious hieroglyphics. They will certainly be of little use to us so long as we persist in regarding them as vegetable forms, and until we have very carefully studied all kinds of modern markings.[150] Nor does it seem of much use to assign to them specific names. The same trail often changes from one so-called species, or even genus, to another in tracing it along, and the same animal may in different circumstances make very different kinds of tracks. There will eventually, perhaps, arise some general kind of nomenclature for these markings under a separate sub-science of Ichnology or the doctrine of Footprints. [150] Geologists are greatly indebted to Dr. Nathorst of Stockholm for his painstaking researches of this kind. I have said nothing of true Algæ or seaweeds, of which there are many fossil species known to us by their forms, and also by the carbonaceous or pyritous matter, or discharge of colour from the matrix, which furnishes evidence of the presence of organic material; nor of the marks and trails left by seaweeds and land plants drifting in currents, some of which are very curious and fantastic; nor of those singular trails referred to the arms of cuttle-fishes and the fins of fishes, or to sea jellies and starfishes. These might form materials for a treatise. My object here is merely to indicate the mode of dealing with such things, and the kind of information to be derived from them.