have left most curious traces of their presence in the trails which they have produced. Some of the most ancient sandstones have their surfaces covered with rows of punctured impressions (_Protichnites_, first footprints), others have strange series of transverse grooves with longitudinal ones at the side (_Climactichnites_, ladder footprints); others are oval burrows, marked with transverse lines and a ridge along the middle (_Rusichnites_, wrinkle footprints). All of these so nearly resemble the trails and tracks of modern king-crabs that there can be little doubt as to their origin. Many curious striated grooves and bifid marks, found on the surfaces of Primordial beds, and which have been described as plants, are probably only the marks of the oral organs or feet of these and similar creatures, which passed their lives in grubbing for food in the soft, slimy ooze, though they could, no doubt, like the modern king-crabs, swim when necessary. Some still more shrimp-like creatures, Hymenocaris, which are found with them, certainly had this power. [F] Woodward has recently suggested affinities of Trilobites with the Isopods or equal-footed crustaceans, on the evidence of a remarkable specimen with remains of feet described by Billings. A lower type of annulose or ringed animal then that of the Trilobites, is that of the worms. These creatures cannot be preserved in a fossil state, except in the case of those which inhabit calcareous tubes: but the marks which their jointed bodies and numerous side-bristles leave on the sand and mud may, when buried under succeeding sediments, remain; and extensive surfaces of very old rocks are marked in this way, either with cylindrical burrows or curious trails with side scratches looking like pinnate leaves. These constitute the genus _Crusiana_, while others of more ordinary form belong to the genus _Arenicolites_, so named from the common Arenicola, or lobworm, whose burrows they are supposed to resemble. Markings referable to seaweed also occur in the Primordial rocks, and also some grotesque and almost inexplicable organisms known as _Oldhamia_, which have been chiefly found in the Primordial of Ireland. One of the most common forms consists of a series of apparently jointed threads disposed in fan-like clusters on a central stem (_Oldhamia antiqua_). Another has a wider and simpler fan-like arrangement of filaments. These have been claimed by botanists as algæ, and have been regarded by zoologists as minute Zoophytes, while some more sceptical have supposed that they may be mere inorganic wrinklings of the beds. This last view does not, however, seem tenable. They are, perhaps, the predecessors of the curious _Graptolites_, which we shall have to represent in the Silurian.