History:

Psoriasis can be highly variable in morphology, distribution, and severity.  The lesions of chronic plaque psoriasis are classically very well circumscribed, circular, red papules or plaques with a grey or silvery-white, dry scale.  Despite the classic presentation, the morphology can range from small tear shaped papules (guttate psoriasis) to pustules (pustular psoriasis) and generalised erythema and scale (erythrodermic psoriasis).  The terms rupioid and ostraceous relate to distinct morphological subtypes of plaque psoriasis.  Rupioid plaques are small (2–5 cm in diameter) and highly hyperkeratotic, resembling limpet shells.  Ostraceous psoriasis refers to hyperkeratotic plaques with relatively concave centres, similar in shape to oyster shells.

^Ref:

Langley RG, Krueger GG, Griffiths CE. Psoriasis: epidemiology, clinical features, and quality of life. Ann Rheum Dis. 2005 Mar;64 Suppl 2:ii18-23; discussion ii24-5.