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Trinucleidae
The Trinucleidae are a family of small to average size trilobites that first occurred at the start of the Ordovician and went extinct at the end of that period. It contains approximately 227 species divided over 51 genera in 5 subfamilies. The most conspicuous character is the wide perforated fringe of the head.
== Description ==
Trinucleidae have a relatively large headshield, that is characterized by a highly vaulted pear-shaped glabella, separated by deep furrows from the cheeks that are often vaulted too, but less so. To the front and sides of this tri-nucleate centre is a wide seam (or fringe), that often inclinates towards the outer margin, and is perforated by funnel shaped pits. These pits are only in one row at the front in the first occurring genera, irregularly distributed in early species, but in later species these are largely arranged in several rows parallel to the margin, and in arches more or less perpendicular to the margin. The thorax consists of six short but wide segments, the outline of which is a continuation of the cheeks. The axis is narrow, less than ⅓× as wide as each of the ribs (or pleurae) to its sides. The tailshield (or pygidium) is small, approximately wide triangular, with the axis narrow triangular, reaching the border. The axial rings and ribs become less conspicuous further to the back.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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