1. Mouth in the anterior half, Genus *_Frontonia_
undulating membrane on left edge
only; right edge continued in a
long ventral furrow

2. Two undulating membranes; mouth Genus *_Colpidium_
central; no caudal bristles

3. Two undulating membranes; caudal Genus *_Uronema_
bristle


Genus FRONTONIA Ehr. (Cl. & Lach. '58?).

(Ehrenberg, subgenus of _Bursaria_ '38; Claparède & Lachmann '58;
Bütschli '88; Shevyakov '96.)

Form elongate and cylindrical, or often flattened dorso-ventrally,
with round or pointed ends. It is usually plastic and contractile.
Cilia are evenly distributed about the body and are similar in
length. The large, open mouth lies on the anterior half of the
ventral surface, and is elongate and oval in outline. On its left
edge is a well-defined membrane which stretches across to the right
side of the mouth. On the right edge is a small, longitudinally
striped tract which is free from trichocysts and smooth in
appearance. This tract is continued posteriorly in a long furrow,
which in some cases reaches the posterior end of the animal. A few
rows of cilia in this furrow vibrate differently from the others and
give the effect of a membrane (Bütschli). The oesophagus is extremely