The diatom genus Tetracyclus Ralfs is largely composed of extinct, freshwater species many of which have been used as stratigraphic markers for several continents under the assumption that they are relatively widespread and confined to precise geological epochs.
The taxonomy of the genus has to a greater extent relied almost exclusively on the shape and dimensions of the siliceous valves and the frequency of particular valve characters such as striae and rib density. In previous studies it has been established that the genus has around 50+ species of which only four have been reported living and at least 30 reported exclusively as fossils. The first monograph on Tetracyclus was by Hustedt (1914), primarily undertaken to provide illustrations for Schmidt’s Atlas der Diatomaceenkunde (Schmidt et al. 1874-1959). Williams (1995) divided the genus into five informal groups based upon the more typical identifying feature(s) of the valve, such as its overall shape, in line with previous work, of which one was the ‘ellipticus’ species group. No phylogenetic significance was attached to these groups.
This checklist is a guide to the information on described species in the genus and is intended for use by stratigraphers, taxonomists and (paleo)biogeographers.
Type species: Tetracyclus glans (Ehrenb.) Mills 1935, p. 1602 = Tetracyclus lacustris Ralfs 1843, p. 105, pl. 2, fig. 2.
Biblarium Ehrenb. 1843, p. 47, nom. rej.
Type: Biblarium glans (Ehrenb.) 1843 = Navicula glans Ehrenb. 1838.
Rejected vs. Tetracyclus Ralfs 1843 (Williams 1986; see ICBN 1994, p. 123). According to the Catalogue of Diatom Names (12 December 2007), lectotypification of Biblarium with B. glans was first initiated by Ross in Farr et al. (1979), p. 200.
Stylobiblium Ehrenb. 1845, p. 72.
Lectotype (Round et al. 1990, p. 703): Stylobiblium clypeus Ehrenb. 1845.
Salacia Pant. 1889, p. 68 non Salacia L. 1771, p. 159.
Type: Salacia boryana Pant. 1889
Gomphogramma A. Braun in Rabenh. 1853, p. 33.
Type: Gomphogramma rupestre (Kütz.) A. Braun in Rabenh. 1853 = Denticula thermalis var. rupestris Kütz. 1849
Hylobiblium Wolle 1894, pl. 22, fig. 5, invalid
Type: Biblarium (Hylobiblium) eccentricum (Ehrenb.) Wolle = Stylobiblium excentricum Ehrenb. 1845.
According to the Catalogue of Diatom Names (12 December 2007), the name Hylobiblium might be a typographic error for Stylobiblium.
Castracania De Toni 1892, p. 750.
Type: Castracania boryana (Pant.) De Toni 1892 = Salacia boryana Pant. 1889.
? Prae-epithemia Jousé 1952, p. 243.
Type: Prae-epithemia robusta Jousé 1952.
Original Description: “Filaments free, four-sided, each side rounded and forming the segment of a distinct circle; frustules longitudinally striated” (Ralfs 1843, p. 105).
Frustules usually rectangular in girdle view, when mature pervalvar axis exceeding apical axis. Valves isopolar, their surface flat, mantle vertical, margin a sharply stepped hyaline lip. Primary internal transapical ribs present, sometimes with secondary and tertiary ribs. Axial area longitudinal with indefinite margins, often internally thickened forming a sternum. Striae parallel to slightly radiate, extending across mantle to the projecting hyaline valve margin. Virgae coarser than vimines, raised above valve surface close to margin, becoming less so towards sternum. Areolae close to sternum more widely spaced than elsewhere, and somewhat irregularly distributed. Apical pore fields present in some species, consisting of a demarcated area with striae closely spaced, as are the areolae within them. Rimoportulae often present, simple, varying in number and position. Cingulum usually consisting of several open bands, opening at alternate poles; others with closed bands. Cingulum composed of four kinds of bands: valvocopula, bearing a septum; primary copulae, each with ligula and septum; secondary copulae, without septa, their ligulae larger than those of the primary copulae; a single pleura. Septa arising from the abvalvar part of the pars exterior of valvocopula and primary copulae, extending variable distances into frustule interior; pore situated at or near base of septum, on abvalvar face communicating with a group of pores on the outer face of the band.