along the left side of the peristome, and is usually
straight.

1. Body cylindrical; size medium; Genus _Metopus_
peristome long and turns sharply
to the left at the extremity

2. No torsion in the peristome; Genus _Blepharisma_
undulating membrane is confined
to the posterior part of peristome

3. No peristomial torsion; Genus _Spirostomum_
body highly contractile;
no undulating membrane

KEY TO THE MARINE GENERA OF BURSARIDAE.

Diagnostic characters: The body is usually short and pocket-like, but
may be elongate. The chief characteristic is the peristome, which is
not a furrow, but a broad triangular area deeply insunk and ending in
a point at the mouth. The adoral zone is usually confined to the left
peristome edge, or it may cross over to the right anterior edge.

1. The anterior half of the body Genus _Balantidium_
tapers to nearly a point in front;
the peristome is narrowest at the
apex; the mouth is the entire
peristome base.

2. The anterior end does not taper; Genus *_Condylostoma_
the peristome is widest at the end
of the body; the mouth is clearly
defined.

* Presence at Woods Hole indicated by asterisk.

Genus CONDYLOSTOMA (KONDYLOSTOMA Bory de St. Vincent 1824) Dujardin
'41

(Dujardin '41; Claparede & Lachmann '58; Stein '59, '67; Cohn '66;
Quennerstedt '67; Wrzesniowski '70; Buetschli '76, '88; Kent '81;
Maupas '83; Shevyakov '96.)

Colorless and more or less flexible animals of medium size. The
general form is elongate and cylindrical or somewhat smaller
anteriorly. The posterior end is broadly rounded, the anterior end
somewhat truncate and oblique. The peristome is broad and triangular,
the base of the triangle being the entire anterior end of the body.
The entire length of the peristome is one-fourth or less of the body
length. The mouth is large and placed at the apex of the peristomial
triangle and opens into a comparatively small oesophagus. The right
edge of the peristome is lamellate and bears a clearly defined
undulating membrane. The adoral zone is well developed upon the left
edge of the peristome, from which it passes around anteriorly to the
right edge. The surface of the peristome is free from cilia, but
the rest of the body is uniformly coated with small active cilia.
Contractile vacuoles are not safely determined. Buetschli thinks
there is probably one terminal vacuole, but som