Prionocheilus Rouault, 1847

Type species. Prionocheilus verneuli Rouault, 1847, by monotypy.

Discussion. We accept the priority of the generic name Prionocheilus over Pharostoma (see Dean 1971). Reed (1915, p. 46) erected a species based on a cranidium, Calymene (Pharostoma) liluensis, which might be regarded as a Prionocheilus species. Zhou et al. (1977) erected the genus Xuanenia, type species X. pustulosus Zhou, 1977, from central South China, based on an imperfectly preserved cranidium. The same genus and species were subsequently described by Tripp et al. (1989) for material from the Upper Ordovican Tangtou Formation, Jiangsu Province. The similarity of some of this material to ‘ Pharostoma ’ liluensis was noted already by Fortey & Cocks (1998), and is confirmed by the present study. Tripp et al. (1989) remarked that “ Xuanenia is extremely close to Prionocheilus Roualt, 1847 (sic)[and] differs only in the effaced lateral glabellar lobes and furrows and smaller pygidium with fewer segments.” There is little difference with regard to glabellar lobes and furrows between the cranidium figured by Tripp et al. (1989, fig. 17k) and the type specimen of ‘ Pharostoma ’ liluensis, which in turn is not significantly different from cranidia of Prionocheilus spp.; we prefer to use that generic name pending further clarification of Xuanenia . It is possible that P. liluensis has priority over subsequently named species in this group, but a comparative study of adjacent species is needed to clarify this.