Excerpt from Some Eocene Diatoms From South Atlantic Cores: Part I. New and Rare Species of Arachnoidiscus; Part II. Rutilaria Greville
Cells solitary, valves discoid, with a circular, sub-circular or sub-triangular outline. Valve surface flat or nearly so, but often with the central por tion slightly raised or depressed. Valve surface divided into sectors by radiating costae (furrows or ribs). The primary costae proceed from the margin and almost reach the small circular and often hyaline central area. Between these are shorter secondary costae that usually penetrate the valve to a distance equal to about half the radius or less, while others, the tertiary costae, are usually much shorter and often restricted to the marginal zone as an elongated punctum. Valve surface of the inter-costal sectors areolate, areolae moderately large, usually sub-quadrate, sub-circular or sub-triangular, frequently more or less uniform in size though a little reduced asthey approach the valve margin. Areolae usually in continuous concentric circles, but in some species the lines of areolae on one sector alter nate with those on contiguous sectors. When the valve is intact, the areolae are furnished with dendritic outgrowths or volae that partially oc clude the aperture through the valve. In many fos sil species these structures are missing due to erosion. In some species a hexagonal system of ridges is superimposed upon the walls of the areolae, often these are eroded to provide an ir regluar pattern. Sometimes the walls of the areo lae are verrucose bearing small pustules at the intersection of the walls; in others fine costae or a series of reticulate lines are also arranged con centrically linking the primary radial costae and anastomosing over the valve surface giving it the appearance of a spider's web. Arachnoidiscus is usually found sessile on some of the smaller red algae that gather around the holdfasts of the larger kelps, and appears to favour the temperate and sub-tropical zones.
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