Before leaving the Lingulæ, I may mention that Mr. Morse informs me that living specimens, when detached from their moorings, can creep like worms, leaving long furrows on the sand, and that they can also construct sand-tubes wherein to shelter themselves. This shows that some of the abundant "worm burrows" of the Primordial may have been the work of these curious little shell-fishes, as well as, perhaps, some of the markings which have been described under the name of _Eophyton_, and have been supposed, I think incorrectly, to be remains of land plants. In addition to Lingula we may obtain, though rarely, lamp-shells of another type, that of the Orthids, These have the valves hinged along a straight line, in the middle of which is a notch for the peduncle, and the valves are often marked with ribs or striae. The Orthids were content with limestone for their shells, and apparently lived in the same circumstances with the Lingulæ; and in the period succeeding the Primordial they became far more abundant. Yet they perished at an early stage of the world's progress, and have no representatives in the modern seas. In many parts of the Primordial ocean the muddy bottom swarmed with crustaceans, relatives of our shrimps and lobsters, but of a form which differs so much from these modern shell-fishes that the question of their affinities has long been an unsettled one with zoologists. Hundreds of species are known, some almost microscopic in size, others a foot in length. All are provided with a broad flat horseshoe-shaped head-plate, which, judging from its form and a comparison with the modern king-crabs or horseshoe-crabs, must have been intended as a sort of mud-plough to enable them to excavate burrows or hide themselves in the slimy ooze of the ocean bed. On the sides of this buckler are placed the prominent eyes, furnished with many separate lenses, on precisely the same plan with those of modern crustaceans and insects, and testifying, as Buckland long ago pointed out, to the identity of the action of light in the ancient and the modern seas. The body was composed of numerous segments, each divided transversely into three lobes, whence they have received the name of _Trilobites_, and the whole articulated, so that the creature could roll itself into a ball, like the modern slaters or wood-lice, which are not very distant relatives of these old crustaceans.[F] The limbs of Trilobites were long unknown, and it was even doubted whether they had any; but recent discoveries have shown that they had a series of flat limbs useful both for swimming and creeping. The Trilobites, under many specific and generic forms, range from the Primordial to the Carboniferous rocks, but are altogether wanting in the more recent formations and in the modern seas. The Trilobites lived on muddy bottoms, and their remains are extremely abundant in shaly and slaty beds, though found also in limestone and sandstone. In the latter they