The only known Cambrian Arthropods are the Crustacea, and of these much the most abundant group is that of the Trilobita, which are altogether confined to the Palaeozoic rocks, and are by far the most important of Cambrian fossils. It is only within recent years that the systematic position of the Trilobites has been established through the fortunate discovery of specimens with their appendages attached (see PI. VII, Figs, 1, 1 a). Trilobites have a more or less distinctly three-lobed body, at one end of which is the head-shield, usually with a pair of fixed compound eyes; at the other end is the tail-shield, and between the two shields is a ringed or jointed body made up of a variable number of movable segments. The Trilobites display an extraordinary variety in form and size, in the proportions of the head- and tail-shields, in the number of free segments, and in the development of spines. Already in the Cambrian this wealth of forms is notable, though far less so than it became in the Or-dovician. As compared with those of later times, the Cambrian Trilobites are marked by the (usually) very small size of the tail-shield, the large number of free segments, and their inability to roll themselves up.

The large Trilobites with long eye-lobes are very distinctively Cambrian. Some of them, like Paradoxides (III, 6), are very large (from 10 inches to 2 feet in length). Olenellus (II, 2) and Holmia (II, 1) also have large species, while Agnostus (III, g),Microdiscus (III, 10), and Atops (III, 8) are small and without eyes.

Plate II.   Cambrian Trilobites.

Plate II. - Cambrian Trilobites.

Fig. i, Holmia broggeri Wale, x 1/2, L. C. 2, Olenellus thompsoni Hall, x 1/2 L. C. (Walcott).

Plate III.   Cambrian Crustacea.

Plate III. - Cambrian Crustacea.

Fig. i, Ptychoparia kingi Meek, x 1/2 M. C. 2, P.antiquitata Salter, x 1/2,M. C. 3, Crepicephalus texanus Shumard, x 1/2, M. C. 4, Mesonacis vermontana Wale, x 1/2, L. C. s, Zacanthoides typicalis Wale, x 1, M. C. 6, Paradoxtdes harlani Green, x 1/4 M. C. 7, Dorypyge curticei Wale, x 1/2 M. C. 8, Atops trilineatus Emmons,x 1/2, L. C. 9, Agnostus interstrictus White, x 3/2, M. C. 10, Microdiscus spectosus Ford, x 1, L. C. 11. Hipponicharion eos Matthew, x 4, L. C. 12, Aristozoe rotundata Wale.

The great importance of the Trilobites for Cambrian stratigraphy is indicated by {he fact that the three divisions of the system are named for the three dominant genera of these crustaceans, Olenellus, Paradoxides, and Dikellocephalus.

Two other divisions of the Crustacea are found in the Cambrian: the Ostracoda, little bivalve forms, whose shells look deceptively like those of molluscs; and the Phyllocarida, which have a large shield on the head and thorax, and a many-jointed abdomen, with terminal spine.