New camera lucida techniques used in the measurement of illaenid trilobites are described and allow quantification of convexity of cephala and pygidia. Application of these techniques has demonstrated
that previously identified dimorphs of Stenopareia glaber are only end members of continuous variation
and that the species ranges from the upper Caradoc well into the Ashgill. A quantitative and qualitative
distinction between S. glaber and S. linnarssoni is presented together with descriptions of Illaenus
( Parillaenus) roemeri and specimens termed I. (P.) cf. dalecarlicus. The latter throws light on the possible
relationship between Octillaenus and other illaenids. Comparative material from Sweden is discussed
and figured as is new material of Panderia from Norway.