EFFACED STYGINID TRILOBITES FROM THE SILURIAN OF NEW-SOUTH-WALES

Eight species of illaenimorph trilobites belonging to five genera of t he Styginidae are described from limestones of the mid-late Wenlock to Ludlow Mirrabooka Formation and its stratigraphical equivalents in th e Orange district, New South Wales. The morphology of illaenimorph (= effaced) styginids is discussed; the term 'omphalus' is introduced for the socketed, tubercle-like projection present in some genera on the interior of the cranidium at or in front of the anterior end of the ax ial furrow. Amongst other characters, the gross convexity of the exosk eleton, the form of the rostral plate, the presence of the omphalus, t he form of the thorax, and possibly the form of the hypostome are deem ed most useful for generic diagnosis; characters used for discriminati on at a lower taxonomic level include the proportions of the exoskelet on, the degree of effacement, the pattern of cranidial muscle scars, t he size and position of the eye, and the character and disposition of sculpture. New taxa are Excetra iotops gen. et sp. nov., Lalax olibros gen. et sp. nov., L. lens gen. et sp. nov., Rhaxeros synaimon sp. nov . and R. trogodes sp. nov. Bumastus (Bumastella) Kobayashi and Hamada is raised to generic status and its diagnosis emended; specimens from New South Wales are assigned to the type species B. spicula, which is considered to be synonymous with five other Japanese species assigned to three different genera by Kobayashi and Hamada. Bumastus is tentati vely recorded on the basis of a single rostral plate; the genus is oth erwise known with certainty only from Laurentia and eastern Avalonia. Meraspid transitory pygidia of Bumastella and Lalax from New South Wal es are up to eight times larger than those of other styginids with wel l documented ontogenies; transitory pygidia of large size are known al so in some other Silurian effaced styginids, and it is suggested that the phenomenon may result from neoteny. The assumption that sexual mat urity in trilobites coincided with the meraspid-holaspid transition is refuted. The effaced styginids from New South Wales show strong fauna l affinity with those from the Upper Wenlock or Lower Ludlow of Japan.