- The Xiphosura are marine in habit, frequenting the shore.
They may be regarded as " macrourous " Xiphosura; that is to say, Xiphosura in which the nomomeristic number of eighteen From Zittel's Palaeontology.
Orders: Xiphosura, Gigantostraca.
However different in structure Trilobites may be, they all agree in possessing a head-shield usually semi-circular in shape, which results from the fusion of apparently five segments, and bears, except in some blind forms, a pair of large reniform compound eyes like those of the king-crab (Xiphosura).
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