Idiomesus Raymond, 1924 has been considered to be a Late Cambrian member of the blind family Shumardiidae on the basis of its small size, absence of eyes, effacement, and marginal sutures. However, evaluation of new material, including previously undescribed species, demonstrates that Idiomesus is a member of the ptychaspidid subfamily Ptychaspidinae. The genus now includes both sighted and blind species, as well as furrowed and effaced species with both marginal and opisthoparian sutures. New species are Idiomesus granti, I. greggi, and I. ultimus.

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