In defining the different species of the genus encrinurus (Emmrich) use has frequently been made of the disposition of nodes on the rings of the mid-lobe of the tail-shield. It is largely by this diagnostic that Foerste, for example, distinguishes E. thresheri from E. ornatus. Hall and Whitfield and the latter authors again, use the same criterion in separating E. ornatus from the European species figured in Murchison\u27s Siluria
