Diagnostic characters:  The peristome is a narrow furrow which begins, as a rule, close to the anterior end and runs backward along the ventral side, to the mouth, which is usually placed between the middle of the body and the posterior end.  A well-developed adoral zone stretches 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 some observers