The confused taxonomic history of the Ludlow graptolite species Neodiversograptus nilssoni (Barrande, 1850) is reviewed and shown to be unstable, according to the International Code of Zoological Nomenclature, since there is no valid holotype. Moreover, recent work has also shown that the previous conception of the species included two separate taxa—N. nilssoni and Cucullograptus (Lobograptus) progenitor. As a result, an application has been made to the International Commission on Zoological Nomenclature to rationalize the situation and designate a neotype. Previous records of ‘Monograptus nilssoni’ in the British Isles have been found to be referable to L. progenitor, and N. nilssoni is now recorded for the first time in this area. Thus the importance of recognising this basal Ludlow graptolite zone is emphasised. On the basis of this new material from the Long Mountain, Welsh Borderland and North Wales, which includes several complete rhabdosomes in both monograptid and diversograptid (bipolar) stages of growth, some preliminary observations are made on the regularity of development of the sicular cladium in this species. Systematic descriptions of both N. nilssoni and L. progenitor are included.