AN EMENDED DESCRIPTION OF DECUSSATA (PATRICK) LANGE-BERTALOT & METZELTIN THAT INCLUDES PROTOPLAST ORGANIZATION AND DETAILED VALVE AND CINGULUM ULTRASTRUCTURE

Abstract

The subgenus DECUSSATA R.M. Patrick was recently elevated to generic status as Decussata (R.M. Patrick) H. Lange-Bertalot & Metzeltin in Lange-Bertalot (2000. Iconographica Diatomologica, 9, 670–673) to accommodate Navicula placenta Ehrenberg and its allies. The description of Decussata is based on characteristics of the valve: solitary cells, flat, broadly elliptic valves with various ends, and areolae with circular foramina arranged in regular quincunx patterns crossing at 60°-80°. A collection from Cedar Hills Sand Prairie in Black Hawk Co., Iowa (U.S.A.), had abundant live Decussata placenta (Ehrenberg) Lange Bertalot & Metzeltin in Lange-Bertalot, the generitype, and a collection from near Prava Reka, Macedonia, had numerous specimens of Decussata hexagona (Torka) Lange-Bertalot. These collections permitted a first observation of living Decussata and a detailed study of their valve and cingulum ultrastructure. Based on this material we place Decussata in the Order Mastogloiales and Family Mastogloiaceae, and provide an emended description of the genus to include details of valve ultrastructure including structure of the cingulum, areolae occluded by internal convex hymenes, and the protoplast characters of a central nucleus, two chloroplaste per cell, one toward each pole, H-shaped in girdle view, variably invaginated in valve view, with a pyrenoid in the plastid bridge.