Order 2. ODONTOSTOMATIDA Sawaya, 1940

(for Ctenostomata, Ctenostom[at]ida, Ctenostomina; syn. Odontostomata)

Body small, laterally compressed, wedge- or helmet-shaped (nearly as wide as long), with armor-like cuirass and often short posterior spines or, occasionally, cirri; somatic ciliature reduced; buccal membranelles inconspicuous, < 10 in number; one to several rounded macronuclei; no cytoproct; fresh-water (rarely marine) polysaprobic forms.

"Chunky" forms, generally with short posterior spines and no cirri; somatic ciliature relatively dense (in its short anterior and posterior kinetal segments); some species with well-developed "tooth" (spine) overhanging hidden buccal cavity; macronuclear number variable; found in polysaprobic habitats, principally fresh-water, but a few Epalxella in marine biotopes.	Family EPALXELLIDAE Corliss, 1960
Body discoidal, smooth in outline except for prominent anterior spine(s) and protruding "ciliary stripe" overhanging buccal cavity; posteriorly, several conspicuous cirri and a single spine; macronucleus single; found in fresh water, under polysaprobic conditions.	Family DISCOMORPHELLIDAE Corliss, 1960
Body discoidal, with very sparse somatic ciliation; posterior end either rounded or with four spines (spineless forms have pair of conspicuous cirri there); macronucleus single; like preceding groups, predominantly in polysaprobic fresh-water habitats (but a few Mylestoma are marine).	Family MYLESTOMATIDAE Kahl in Doflein & Reichenow; 1929