• Peridinium depressum: Bailey (1854)

The cell is thecate,  solitary, of large dimensions,dorso-ventrally compressed. The cell surface is reticulate. The circular cingulum descends by about twice its width and divides the cell into two parts: the epitheca, which has concave sides and a prominent apical horn, and the hypotheca, which has two long divergent antapical horns. The sulcus is very deep. The apical plate 1’ has the "ORTHO" shape, while the 2^nd apical intercalary plate is "quadra". The cytoplasm has pink granulations, and in the centre there is a large nucleus.

Cosmopolitan, coastal and oceanic, typical of temperate to tropical waters.

Distribution: as Peridinium depressum: Ireland: Galway (Boyd, 1973).
Europe: Ireland (Boyd, 1973), Romania (Caraus, 2002).

As Protoperidinium depressum: Europe: Great Britain (Parke & Dixon, 1976), Ireland (Gribble & Anderson, 2006, Gribble & Anderson, 2007); Atlantic Islands: Canary Islands (Gil-Rodríguez et al., 2003); North America: Mexico (Okolodkov, 2005), Nova Nova Scotia (Kim, Garbary & McLachlan, 2004); Central America: Belize (Faust, 2003).

The algae of Romania., Studii si Cercetari, Universitatea Bacau, Biologie 7, Caraus I., 2002, 1-694

Plankton species, some new to the area, obtained in Galway Bay by use of a plankton pump., Irish Naturalists' Journal 17:, Boyd R.J., 1973, 330-335

Phytoplankton dynamics in Pomquet Harbour, Nova Scotia: a lagoon in the southern Gulf of St., Lawrence. Phycologia 43, Kim K.Y., Garbary D.J., McLachlan J.L., 2004, 311-328.

Protoperidinium belizeanum sp. nov. (Dinophyceae) from Manatee Cay, Belize, Central America (Note)., Journal of Phycology 39:, Faust M.A., 2003, 390-394

Check-list of British marine algae - third revision., Journal of the Marine Biological Association of the United Kingdom 56, Parke M., Dixon P.S., 1976, 527-594.

El genero "Protoperidinium" Bergh, 1881 ("Peridinium" Ehrenberg, 1831, partim)., Revta Mus. argent. Cienc. nat. "B. Rivadavia". Hidrobiol. 3:, Balech E., 1974, 347-368.

Protoperidinium Bergh (Dinoflagellata) in the southeastern Mexian Pacific Ocean: part I., Botanica Marina 48:, Okolodkov Y.B., 2005, 284-296

High intraindividual, intraspecific, and interspecific variability in large subunit ribosomal DNA in the heterotrophic dinoflagellates Protoperidinium, Diplopsalis, and Preperidinium (Dinophyceae)., Phycologia 46(3):, Gribble K.E., Anderson D.M., 2007, 315-324

Molecular phylogeny of the heterotrophic dinoflagellates, Photoperidinium, Diplopsalis and Preperidinium (Dinophyceae), inferred from large subunit rDNA., Journal of Phycology 42:, Gribble K.E., Anderson D.M., 2006, 1081-1095

Proctoctista. In: Lista de especies marinas de Canarias (algas, hongos, plantas y animales). (Moro, L., Martín, J.L., Garrido, M.J. & Izquierdo, I. Eds), Las Palmas: Consejería de Política Territorial y Medio Ambiente del Gobierno de Canarias., Gil-Rodríguez M.C., Haroun R., Ojeda Rodríguez A., Berecibar Zugasti E., Domínguez Santana P., Herrera Morán B., 2003, 5 - 30