Genus Trinema Dujardin, 1841
Diagnosis: test composed of two types of scales, large circular plates and smaller circular or broad elliptical plates in between or overlapping; aperture subterminal, ventral, circular or broad oval, invaginated, with rows of toothed apertural plates; apertural rim with oval small scales. Nucleus vesicular. Filopodia.
Ecology: very common in water, between mosses and in soil.
Remarks: species are very variable in size and shape and hard to identify.
Key to the species:
|1||Aperture circular with a thin border; length of test less than twice the breadth||2|
|-||Aperture oval with a thick border; length of shell at least twice the breadth||4|
|2||Test small elliptical, length usually < 40 um||T. lineare|
|-||Test broader, elliptical to ovoid; length usually > 45 µm||3|
|3||Test < 80 µm||T. enchelys|
|-||Test > 100 µm||T. grandis|
|4||Anterior end very compressed with a thick anterior lip; aperture slightly invaginated||T. complanatum|
|-||Test in side view ovoid; aperture distinctly invaginated, with broad bend anterior lip||T. penardi|