material has been drifted away by the wind; the portion consolidated by pressure being better able to resist the denuding agency. [P] "Impressions and Footprints of Aquatic Animals," "American Journal of Science," 1873. [Illustration: Fig. 5.--Trail of a modern king-crab, to illustrate imitations of plants sometimes named _Bilobites_.] [Illustration: Fig. 6.--Trail of Carboniferous crustacean (_Rusichnites Acadicus_), Nova Scotia, to illustrate supposed Algæ.] The footprints from the Potsdam sandstone in Canada, for which the name _Protichnites_ was proposed by Owen, and which were by him referred to crustaceans probably resembling _Limulus_, were shown by the writer, in 1862,[Q] to correspond precisely with those of the American Limulus (_Polyphemus Occidentalis_) (Fig. 5). I proved by experiment with the modern animal that the recurring series of groups of markings were produced by the toes of the large posterior thoracic feet, the irregular scratches seen in _Protichnites lineatus_ by the ordinary feet, and the central furrow by the tail. It was also shown that when the Limulus uses its swimming-feet it produces impressions of the character of those named _Climactichnites_, from the same beds which afford _Protichnites_. The principal difference between _Protichnites_ and their modern representatives is that the latter have two lateral furrows produced by the sides of the carapace, which are wanting in the former. [Q] "Canadian Naturalist," vol. vii. I subsequently applied the same explanation to several other ancient forms now known under the general name _Bilobites_ (Figs. 6 and 7).[R] [R] The name Bilobites was originally proposed by De Kay for a bivalve shell (Conocardium). Its application to supposed Algæ was an error, but this is of the less consequence, as these are not true plants but only animal trails. [Illustration: Fig. 7.--_Rusophycus_ (_Rusichnites_) _Grenvillensis_, an animal burrow of the Siluro-Cambrian, probably of a crustacean, _a_, Track connected with it.] The tuberculated impressions known as _Phymatoderma_ and _Caulerpites_ may, as Zeiller has shown, be made by the burrowing of the mole-cricket, and fine examples occurring in the Clinton formation of Canada are probably the work of Crustacea. It is probable, however, that some of the later forms referred to these genera are really Algæ related to _Caulerpa_, or even branches of Conifers of the genus _Brachyphyllum_. _Nereites_ and _Planulites_ are tracks and burrows of worms, with or without marks of setæ, and some of the markings referred to _Palæochorda_, _Palæophycus_, and _Scolithus_ have their places here. Many examples highly illustrative of the manner of formation of the impressions are afforded by Canadian rocks (Fig. 8).