The dorsal (back) surface region of a trilobite's body that lacked certain typical structures.
From Latin 'a-' (without) + 'notum' (back/dorsal surface). The '-um' is Latin neuter singular, used in anatomical terminology for body parts.
Trilobite anatomy was so specialized that scientists named specific body regions—the 'notum' is the back, and 'alinotum' specifically describes a back lacking typical ridges or features.
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