Small animals with ellipsoidal form and about twice as long as broad. The mouth lies in the upper half of the body and bears a well-developed undulating membrane upon its left edge. The membrane is longitudinally striped and covered with long and vibratile cilia. The right edge of the mouth bears cilia which are about the same in size as the body cilia, but are more closely inserted (Schewiakoff). The most characteristic feature is the long caudal bristle, which is extremely delicate and about two-thirds the length of the body. Schewiakoff thinks this bristle has a sensory function. I could not make this out, for although other protozoa ran against this bristle, often bending it well over to one side, the animal showed no sign of irritability but lay quiescent. A spherical macronucleus with attached micronucleus lies in the center of the body. The contractile vacuole is posterior in front of the bristle. The macronucleus was found to be double, as though just divided, in a large percentage of cases. This may be a precocious division of the nucleus long before signs of the body division are evident. Such a phenomenon, however, is rare, the macronucleus usually dividing at a late stage of cell division. Length 30 to 50µ; width 15 to 20µ. Common in decomposing algæ. [Illustration: FIG. 39.--_Uronema marina_.] KEY TO MARINE GENERA OF PLEURONEMIDÆ. Diagnostic characters: The mouth is at the end of a long peristome running along the ventral side; the body is dorso-ventrally or laterally compressed. The entire left edge of the peristome is provided with an undulating membrane which occasionally runs around the posterior end of the peristome to form a "pocket" leading to the mouth. The right edge of the peristome is provided with a less-developed membrane. There may or may not be a well-developed pharynx. Body small; not produced into Genus *_Pleuronema_ neck-like elongation Body medium-sized; anterior end Genus *_Lembus_ produced into neck-like elongation * Presence at Woods Hole indicated by asterisk. Genus PLEURONEMA Dujardin '41. (Perty '52; Clap. & Lach. '58; Stein '59, vol. I; Quennerstedt '67; Kent '81; Bütschli '88; Schewiakoff '89; Shevyakov '96.)