Fig. 269. A Phyllopod
During the long ages of the Cambrian the trilobite varied greatly. Again and again new species and genera appeared, while the older types became extinct. For this reason and because of their abundance, trilobites are used in the classification of the Cambrian system. The Lower Cambrian is characterized by the presence of a trilobitic fauna in which the genus Olenellus is predominant. This, the Olenellus Zone, is one of the most important platforms in the entire geological series; for, the world over, it marks the beginning of Paleozoic time, while all underlying strata are classified as pre-Cambrian. The Middle Cambrian is marked by the genus Paradoxides, and the Upper Cambrian by the genus Olenus. Some of the Cambrian trilobites were giants, measuring as much as two feet long, while others were the smallest of their kind, a fraction of an inch in length.
Another type of crustacean which lived in the Cambrian and whose order is still living is illustrated in [Figure 269].
Worms. Trails and burrows of worms have been left on the sea beaches and mud flats of all geological times from the Algonkian to the present.
Fig. 270. A Cambrian Articulate Brachiopod, Orthis
