Abstract
A review of the supposed moulting positions of the Bohemian Ordovician dalmanitoid and acastoid trilobites is presented. Five main moulting positions are described, but some combinations of these are also widespread. The variability of the supposed moulting mechanisms is surprisingly high, especially in Middle Ordovician taxa. All and/or almost all moulting positions may occur together even within a single species. In advanced representatives, only a few of the described mechanisms prevail. Nevertheless, the occurrence of the moulting positions in the Bohemian Ordovician dalmanitoid and acastoid trilobites is considered to be constrained by the overall body morphology, rather than the result of phylogenetical relations. Functionality of the facial suture in these groups during the ontogeny and phylogeny is discussed.