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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.
I here present one of our common Horse-Shoe Crabs above one of these old-world Crustaceans, and it will be seen, that, while the latter preserves some of the Trilobitic characters, such as the marked articulations on the posterior part of the body and their division into three lobes, yet in the prominence of its anterior shield, its more elongated form, and tapering extremity, it resembles its modern representative.
Recent investigations by the naturalists of the Canadian survey have rendered it certain that below the level of the Potsdam Sandstone there are slates and schists extending from New York to Newfoundland, occupied by a series of trilobitic forms similar in genera, though not in species, to those found in the European Upper Cambrian strata.
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