AN ARCHIMEDES-LIKE CYCLOSTOME BRYOZOAN FROM THE EOCENE OF NORTH-CAROLINA

A distinctive spiral unilaminate cyclostome bryozoan, recently discove red in the Eocene Castle Hayne Limestone Formation of eastern North Ca rolina, is described as Crisidmonea archimediformis new species. Colon ies have a thickened spiral axis bearing systems of bifurcating branch es. When worn and stripped of their branches, the resistant axes close ly resemble the familiar screws of the Carboniferous to Permian fenest rate bryozoan genus Archimedes. However, the Eocene screws are shorter and become more loosely coiled with growth, corresponding to a helico spiral rather than a helical form. The paleoenvironment inhabited by C . archimediformis colonies is inferred to have been on the shallow (<3 0 m depth) margin of an open marine, waveswept embayment. Autozooid di stribution and the gradient of increasing apertural size from branch r everse to frontal sides in C. archimediformis resemble those of the li ving cyclostome Exidmonea atlantica and suggest colonial water current s that flowed upwards and inwards through the spiral colony. Polyascos oecia, a genus closely related to Crisidmonea, is shown to be an objec tive junior synonym of Reteporidea, and the new genus Polyascoeciella (type species Idmonea foraminosa Reuss) is introduced for species with exterior-walled gonozooids that were previously assigned to Polyascos oecia.