ascribed to any other creatures with any show of probability. That this is the true nature of some of the markings in question cannot be doubted at all; and in other cases no explanation so probable has yet been suggested. If, however, the tracks which have been described from the Potsdam Sandstone of North America under the name of _Protichnites_ are really due to the peregrinations of some Trilobite, they must have been produced by one of the largest examples of the order. As already said, the Cambrian Rocks are very rich in the remains of Trilobites. In the lowest beds of the series (Longmynd Rocks), representatives of some half-dozen genera have now been detected, including the dwarf _Agnostus_ and the giant _Paradoxides_. In the higher beds, the number both of genera and species is largely increased; and from the great comparative abundance of individuals, the Trilobites have every right to be considered as the most characteristic fossils of the Cambrian period,--the more so as the Cambrian species belong to peculiar types, which, for the most part, died out before the commencement of the Silurian epoch. All the remaining Cambrian fossils which demand any notice here are members of one or other division of the great class of the _Mollusca_, or "Shell-fish" properly so called. In the Lower Cambrian Rocks the Lamp-shells (_Brachiopoda_) are the principal or sole representatives of the class, and appear chiefly in three interesting and important types--namely, _Lingulella, Discina,_ and _Obolella_. Of these the last (fig. 32, i) is highly characteristic of these ancient deposits; whilst _Discina_ is one of those remarkable persistent types which, commencing at this early period, has continued to be represented by varying forms through all the intervening geological formations up to the present day. _Lingulella_ (fig. 32, c), again, is closely allied to the existing "Goose-bill" Lamp-shell (_Lingula anatina_), and thus presents us with another example of an extremely long-lived type. The _Lingulelloe_ and their successors; the _Linguloe_, are singular in possessing a shell which is of a horny texture, and contains but a small proportion of calcareous matter. In the Upper Cambrian Rocks, the _Lingulelloe_ become much more abundant, the broad satchel-shaped species known as _L. Davisii_ (fig. 32, e) being so abundant that one of the great divisions of the Cambrian is termed the "Lingula Flags." Here, also, we meet for the first time with examples of the genus Orthis (fig. 32, f, k, l) a characteristic Palæozoic type of the Brachiopods, which is destined to undergo a vast extension in later ages.