Thaws. Trilobites appear to have been en tirely marine animals, and to have enjoyed vari ous modes of existence. Some were undoubtedly bottom crawlers, others buried themselves in the mud of the bottom, and still others were free swimmers or crawled about coral and hydroid masses after the manner of the modern isopods. The remains of trilobites are usually found in dismembered condition, and entire cara paces are quite rare. This is probably due to the fact that the majority of specimens are the discarded molts which have been broken in the process of shedding. Trilobites abound in all the Cambrian formations, during which period they were in fact the dominant forms of life, and they are abundant in some horizons of the Or dovician and Silurian. During the Devonian they declined rapidly. and only a few genera, represented by rare specimens, continued into the Carboniferous. They are entirely absent from all Mesozoic rocks, and they have no near rela tives at the present day. They are one of the most important groups of fossil animals for purposes of correlation. In all about 2000 spe cies and over 200 genera have been described, the majority of them from the Cambrian and Ordovician rocks.
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