All morphological entities of the cephalon and thorax of the species studied herein indicate that the Olenellidae was probably a highly specialized family of trilobites. Loss of cephalic spines by resorption in which one pair may remain as the genal spines or two pairs remain as the genal and intergenal spines, is evident in specimens of the Olenellidae. The anterior course of the facial suture, even though fused, indicates that this family belongs to the order Opisthoparia. The hypostoma of Olenellus and Paedeumias, although distinct from each other, are of generic and not of specific value. Intergenal spines which occur in O. bristolensis and O. insolens are of generic importance even though they are aborted in early ontogenetic stages. In O. bristolensis, the size of the genal angle, position of the genal spines, and the glabellar ratio vary independently of each other whereas the size of the cephalon displays an orderly progression of growth. Trilobites studied herein do not molt in coordinate stages but molt in an orderly progression regardless of the size of the individual.