An extensive and detailed overhaul of the present classification of the Marrolithinae is undertaken to produce a clear taxonomic framework from which to understand the phylogeny and biogeographical history of the subfamily. Historically, marrolithine species have been defined using a strongly typological approach which has increasingly proved unworkable as species show ranges of variation in morphology that have led to uncertainty in the identification of specimens and has therefore lowered their stratigraphical usefulness. The lack of appreciation of morphological variation has resulted in the naming of a great many more species than are probably actually present in the fossil record. Extensive museum collections have been analysed supplemented by field sampling within Wales and Shropshire and comparison of field locations and horizons. A large collection of over 2000 specimens of Llanvirn to Ashgill Moroccan Marrolithinae was studied for the first time. This material has been important in the reclassification of the Marrolithinae. These samples contain the earliest Onnia specimens and have therefore thrown light on the Deanaspis and Onnia specimens and have therefore thrown light on the Deanaspis and Onnia lineage as presently understood. The Moroccan samples also contain the earliest representatives of the new genus Hammannaspis. The Moroccan species played an important role in the distribution of the Marrolithinae around proto-Tethys. The re-evaluation of the classification of marrolithine genera and species has included the first cladistic analysis of a trinucleid subfamily, detailed analysis of variation in fringe pit development within samples and taxa and consideration of the palaeogeographical relationships between the marrolithine taxa in space and time. These differing approaches have led to the production of a more constrained and easier to use classification which will enhance the stratigraphical usefulness of the Marrolithinae.