D. murchisoni (Beck 1839), the types species of Didymograptus M'Coy, has been the subject of considerable confusion as a result of the extreme taxonomic splitting within the didymograptids. An attempt is made here to show the range of variation in this graptolite, based on the population on the original type slab, and to suggest ways in which studies on populations can lead to better understanding of the stratigraphic range of these Ordovician forms. Gruptolithina, Didymograptus murchisoni, taxonomy.