Tracks and trails of various sorts have been ascribed by authors to trilobites since these problematic markings first began to attract attention, but as the appendages were until recently quite unknown, all the earlier references were purely speculative. The subject is a difficult one, and proof that any particular track or trail could have been made in only one way is not easily obtained. Since the appendages have actually been described, comparatively little has been done, Walcott's work on _Protichnites_ (1912 B, p. 275) being the most important. Since the first description of _Protichnites_ by Owen (Quart. Jour. Geol. Soc., London, 1852, vol. 8, p. 247), it has been thought that these trails were made by crustaceans, and the only known contemporaneous crustaceans being trilobites, these animals were naturally suggested. Dawson (Canadian Nat. Geol., vol. 7, 1862, p. 276) ascribed them, with reserve, to _Paradoxides_, and Billings (1870, p. 484) suggested _Dikelocephalus_ or _Aglaspis_. Walcott secured well preserved specimens which showed trifid tracks, and these were readily explained when he found the legs of _Neolenus_, which terminated with three large spines. Similar trifid terminations had already been described by Beecher, and clearly pictured in his restoration of _Triarthrus_, but the spines and the tracks had somehow not previously been connected in the mind of any observer. Walcott concluded that the tracks had been made by a species of _Dikelocephalus_, possibly by _D. hartti_, which occurs both north and south of the Adirondacks. In a recent paper, Burling (Amer. Jour. Sci., ser. 4, vol. 44, 1917, p. 387) has argued that Protichnites was not the trail of a trilobite, but of a "short, low-lying, more or less heavy set, approximately 12-legged, crab-like animal," which had an oval shape, toed in, and was either extremely flexible or else short and more or less flexible in outline. This seems to describe a trilobite. _Climactichnites_, the most discussed single trail of all, has also been ascribed to trilobites,--by Dana (Manual of Geology, 1863, p. 185), Billings (1870, p. 485), and Packard (Proc. Amer. Acad. Arts and Sci., vol. 36, 1900, p. 64),--though less frequently than to other animals. The latest opinion (see paper by Burling cited above) seems to be against this theory. Miller (1880, p. 217) described under the generic name _Asaphoidichnus_ two kinds of tracks which were such as he supposed might be made by an _Asaphus_ (_Isotelus_). In referring to the second of the species, he says: "Some of the toe-tracks are more or less fringed, which I attribute to the action of water, though Mr. Dyer is impressed with the idea that it may indicate hairy or spinous feet." The type of this species, _A. dyeri_, is in the Museum of Comparative Zoology, and while it may be the trail of a trilobite, it would be difficult to explain how it was produced.