when it walked. While tailless there were simply two parallel sets or rows of toe-tracks, but when tailed those parallel rows were separated by a median line, showing the caudal trail. Is there not here a caution for the interpreters of the "Protichnites" seeing that the same species, at diverse ages, may make widely different tracks? Not more than three or four exuviæ were found entire in the mud of the hatching jars. In all the rest the buckler and the pygidium were separate. Now it is not the case that the Horse Foot shells, in the long wind-rows on the shore-line, are entire. The entire ones are decidedly exceptional. Before certain tides the young are helpless; but the adult never comes shoreward except to spawn. Hence their exuviæ are brought up by the wash and the under-tow of storms, thus effecting the separation of the two parts. Is there not here an explanation of the great abundance of the pygidia, or caudal shields, of the Asaphus Iowensis in the Iowa limestone rocks? I do not regard them as the debris of dead trilobites but as their cast-off shells. They are the tidal windrows of that ancient sea. The articulation of the two carapaces was no doubt feeble; and the specific gravity of the pygidium less than that of the buckler. In this case the debris would be sorted into different depths of water. The bucklers would be less crowded, because in greater depths where the tidal action was less; while the lighter pygidia would, by the same law, form the drift of the shorelines. EXPLANATION OF PLATE 3. Fig. 1. Egg of Limulus just cracked by incubation, showing the pellucid sphere. Fig. 2. Embryo in the egg, much enlarged. Fig. 3. Same two days older, much enlarged. Fig. 4. Young Limulus just out of the egg, enlarged nine diameters. Fig. 5. Terminal tail joint of Pterygotus Banksii. Fig. 6. Terminal tail joint of Pterygotus bilobus. Fig. 7. Terminal tail joint of Pterygotus gigas. AMER. NATURALIST, VOL. IV. 35 Fig. 9. Pterygotus bilobus. Fig. 10. Terminal tail joint of Pterygotus acuminatus. Fig. 11. The smaller one is Limulus just hatched, natural size, mere out. line; the larger is the same undergoing the first moult, and leaving the old shell, and having a tail. Fig. 12. Limulus Polyphemus, one year old. The markings on the posterior carapace become less distinct with adult age. The adult female will attain a size even exceeding twelve inches across the cephalic shield. Fig. 13. Eurypterus remipes; size very much reduced. Fig. 14. Sao hirsutus, a trilobite. THE SEA-WEEDS AT HOME AND ABROAD. BY JOHN L. RUSSELL. THE vegetable productions of the ocean, like those of the drier portions of the earth, are subject to a similar order of distribution. The most common collector of plants becomes soon aware that there are kinds which are not to be looked for in ordinary places, and soon learns to set a value on those which rarely occur to him. He also desires to extend the area of his observations so as to embrace different latitudes, or to obtain the same results by ascending lofty mountain heights. So the collector of sea-weeds does not confine. himself to particular districts, but endeavors, either by personal inspection or else through the labor and courtesy of others, to ascertain what forms, seemingly familiar or entirely diverse, may grow abroad. The deeper soundings of the ocean-beds, like the higher elevations of the land, afford him a greater variety, affected by different causes, which in their natural course produce different results. The general plan of vegetable life, especially in the lower plants, seems to point to constant modification of some one typical form, and this modification appears to have its origin in climatic influences. It becomes a most fascinating study to endeavor to join the separate and divided links so as to possess, in a series of specimens, the probable method of development which nature has thus instituted. Let me endeavor to adapt this idea to the thoughts of this present essay, and arrange to some extent the sea-weeds (Algæ) of our own and of foreign or distant coasts together. Let us see in what kinds there are corresponding ones; and when we select some choice specimen from the beach-drift, or pluck it from the rocks, endeavor to tell on what distant strand it is obedient to the pulsing waves, or perchance attracts other eyes. The coast of New England presents as great a diversity in outline and in character as perhaps can be found in the same length of the Atlantic shore. We have here the deep inlets like Norwegian fiords in Maine; the bold rocky promontories of Massachusetts varied with the almost level and smooth sands of the South. The noblest in size, as well as most beautiful in color and features, are the algae which are to be met with throughout this wide range. The would-be successful collector must resort to the dredging apparatus, and like the shell collector needs a strong arm and abundance of patient toil to serve him; else he must wait some violent storm, which shall break from their deeper moorings those more valuable weeds which only can grow perfectly and develop themselves entirely far below the surface, where the sun's rays but feebly penetrate and the water is of a nearly uniform temperature. Some wonderful waifs are occasionally met with in this way by visiting the beaches and picking over the waste with scrupulous care. In the warmer waters of the Southern States, like those on the Florida Keys, there may be sought singular kinds resembling corals, for which they were formerly mistaken by Lamouroux, some of exquisite beauty in design and shape. Some of these are found growing from the base of a Gorgonia or sea-fan, and secreting from the ocean their covering of lime. And others of richest green creep over the sand beneath the water, and throw up a turf as verdant as that which clothes |