Study of silicified ontogenetic series of opisthoparian (Aphelaspis), gonatoparian (Glaphyraspis), and proparian (Hardyoides) trilobites indicates the probable homology of cephalic spines in ptychopariid trilobites. Opisthoparia, gonatoparia, and proparia are redefined in terms of differential development of cephalic spines relative to a morphologically constant course of the posterior section of the facial suture. Five protaspid instars and a probable growth rate factor of 1.08 can be demonstrated for Aphelaspis.

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