Article: The trilobite Tretaspis from the upper Ordovician of the Oslo region, Norway

All known Norwegian species of Tretaspis are described. Six are established taxa: T. ceriodes (Angelin) angelini Stormer, T. seticornis (Hisinger), T. anderssani Stormer, T. hadelandica hadelandica Stormer, T. sortita (Reed) broeggeri Stormer. and T. kiaeri Stormer. Three are new: T. hisingeri, T. askerensis. and T. latilimbus (Linnarsson) norvegicus. Most of these liixa have a broad runge of variation encompassing two or more morphs. The relative proportions of these morphs are used to distinguish T. latilimbus norvegicus and T. sortita broeggeri from their nominate subspecies. The British form T. convergens Dean and its subspecies are reinterpreted as subspecies of T. hadelandica. Ingham's concept of species groups within Tretaspis is revised with the North American species and, provisionally, T. kiaeri and T. calcaria Dean being recognised as a distinct group centred on T. sagenosus Wellington. Neoteny is considered lo have played a part in the evolution of Tretaspis.