Taxonomy and Valve Ultrastructure of New and Interesting Freshwater Fossil Diatoms (Bacillariophyta) of Miocene age from the Espanola Formation of New Mexico, USA. I. Ontogeny of Tetracyclus stelloides sp. nov., with Comments on ‘Accepted Names’ and Hustedt's Concepts of ‘Convergence’ and ‘Miserable Forms’

We present the size diminution series of a new species of Tetracyclus, described herein as Tetracyclus stelloides Kociolek, D.M.Williams & Danz sp. nov., based on light and scanning electron microscopic observations. The largest cells of T. stelloides sp. nov. are cruciate in outline with broadly rounded apices and central inflations, the smallest cells are elliptical. Size, presence and position of a rimoportula, and valve shape differentiate this species from other described taxa in the genus. We discuss the ontogeny of this cell cycle in the context of Hustedt's concept of convergence and ‘miserable forms’. The taxonomic and nomenclatural implications of these findings for other species of Tetracyclus are discussed, and we explore the confusion and challenges related to the idea of ‘Accepted Names’ in this regard.