Tesselacauda n. sp. A

(Plate 12, figs 13–16, 18–29)

Material. Assigned specimens SUI 147587–147594, from Section MME 36.4m, Fillmore Formation (upper Tremadocian; Stairsian; Rossaspis leboni Zone), southern Confusion Range, Ibex area, Millard County, western Utah.

Discussion. Specimens retrieved from Section MME 36.4 m are from lower in the same zone as those of T. morrisoni . The sample from MME 36.4 m is in general fragmentary and sparse, but most species seem to occur also in the assemblage at MME 49.8 m. The Tesselacauda specimens, however, definitely do not belong to T. morrisoni . The dorsal pygidial surfaces of most specimens are almost completely effaced, and lack the clusters of tubercles on the distal parts of the pleurae which characterize T. morrisoni . The terminal piece, while relatively large, is about as wide as it is long, whereas those of pygidia of T. morrisoni are much longer than wide. The MME 36.4 m material seems to represent a poorly known but distinct species, the oldest member of the genus thus far recovered.