|BGS image ID: P521140|
The exoskeleton of Acernaspis can clearly be seen. It has a central axis and two side regions called pleurae. It is separated from anterior to posterior into a cephalon or head-shield, a thorax and a pygidium or tail-shield. The central region of the cephalon is called the glabella. Note the compound convex eye (like a fly's) made up of a number of separate lenses. This would give them a wide field of view forwards, backwards, sideways and upwards. The trilobites are the earliest of the arthropods. First found in rocks of Lower Cambrian age, they died out in the Permian. Throughout their 350 million year history they maintained a remarkable constancy of form, though with many varieties. All trilobites were marine. Figd. Clarkson, Elderidge, & Henry, pal. 20,plate 19, figs. 12,13.
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