Article: An abnormal specimen of the Silurian anomalocystitid mitrate Placocystites forbesianus

An unusual specimen of the Silurian anomalocystitid mitrate Placocyxiites forbesianus K described. Unlike the normal morphology of the species, the anterior, transverse row of ventral plates consists of two rather than three elements; the mid-ventral plate of the next posterior transverse row is reduced in size and wedged between the lateral elements of the same row; the plate arrangement on the anterior half of the ventral skeleton is asymmetrical; the left lateral and admedian ventral plates are smaller than the corresponding plates of the right side; finally, the placocystid plate has a slightly pronounced anterior angle. The abnormal skeletal features of this specimen are interpreted as a developmental response to the absence of an anterior ventral plate.