Guide to Trieres

Frustules are bipolar, rectangular in girdle view with four tubular processes arising obliquely from an undulate valve surface. Valves with a diagonal symmetry, elliptic-lanceolate to lanceolate in outline. Valve face linear to lanceolate, separated from a tall, deeply sloped mantle by a crenulated or smooth ridge that is uninterrupted except where it meets the two shorter processes near the valve apices. Valve apices subacute and may be slightly produced. The mantle edge may be recurved forming a narrow flange or ledge. Striae bi- or triseriate between weakly developed external costae of the virgae. Areolae loculate, hexagonal, with small, circular internal foramina. Externally, areolae closed by a continuous cribrum with transverse rows of fine pores. The side walls of the loculate areolae are perforated by small pores. Small irregularly-scattered spines may be present amongst the striae. The stria pattern is interrupted along the midline of the valve face where a narrow, sometimes sinuous, elongate annulus is situated. Around the outside perimeter of the annulus, transapical ribs (i.e. the virgae) run continuously across the valve face to the mantle margin. Within the annulus, small circular pores are present in a scattered or circular pattern.

Columnar projections arise from valve face/mantle junction beyond the ends of the annulus supporting an ocellus at each apex. Radial rows of fine pores perforate the ocellus. Two rimoportulae, diagonally placed relative to the transapical axis, occur on the valve face outside the annulus between the two ocelli. The interior opening of the rimoportula is sessile, lip-like, perpendicular to the annulus, and lying in a small depression. The external opening of each rimoportula is at the end of an elongated hollow tube. The apices of this external tube may be simple or forked.

A mature cingulum consists of open, split, ligulate bands with openings at opposite ends of the cell. The valvocopula is wider than the copula. Each band is perforated by transverse rows of hexagonal, loculate areolae. The pars interior of the valvocopula is fimbriate whereas both the pars interior and the pars exterior of the copula are fimbriate. The fimbriate margin of the valvocopula fits into the flange of the mantle.

Living cells contain numerous, small, discoid chloroplasts and the nucleus is centrally placed. The chloroplasts are restricted to the cortical cytoplasm whereas the nucleus is suspended in the central vacuole by cytoplasmic strands. The cells form linear chains to support the planktonic life style of the different species. The cells within a chain are united by mucilage secreted by the ocelli and the overlapping of the diagonally oriented external tubes of the rimoportulae.

Species of Trieres are cosmopolitan in the neritic zones of temperature shallow seas. Ashworth et al. (2013) identified the Eupodiscaceae as a monophyletic group, with the ocellus likely a defining synapomorphy of the group. Within the Eupodiscaceae, they described Trieres Ashworth and Theriot, a monophyletic clade consisting of three species and distinguished from the genus Odontella C.A. Agardh by the presence of loculate areolae. The three species are the generitype T. sinensis (Grev.) M.P.Ashworth and Theriot, T. mobiliensis (Bailey) M.P.Ashworth and Theriot, and T. regia (Schulz) M.P.Ashworth and Theriot. Historically, these species have been variously placed in Odontella, Zygoceros Ehrenb., and Biddulphia Gray.

Morphologically, the most similar genus to Trieres is Parodontella Komura, which is only known from the Miocene. The two genera share features in the structure of the loculate areolae, marginal ridge and tubular processes, but diverge in the shape of the annulus. Rather than an elongate annulus, Komura described Parodontella on the basis of a sternum across the valve face.