Outline of the flattened body ovoid; body consists of two valve-like portions; the edge of the right valve is nearly straight, that of the left valve more or less sinuous; anteriorly it is cut away, obliquely and posteriorly it has a deep indentation in which the seizing spine rests.  The cilia are confined to the ventral surface, here reaching, however, from the anterior dorsal extremity to below the posterior indentation.  Posteriorly the cilia become larger, corresponding to the larger cirri of D. armata, which are posterior to the spine.  The mouth lies between the two valves and is surrounded by a long and smooth buccal armature which passes downward and backward to the left a distance equal to about half the entire body length.  The macronucleus is situated in the dorsal region in the central part of the body.  There are two contractile vacuoles, one behind the center of the buccal armature, the other near the inner end of this organ.  Movement is in circles, the animal moving around quite rapidly when not attached by its posterior process.  It is colorless and measures 45 mu in length by 27 mu in width.  Claparede & Lachmann and Shevyakov describe it as 70 mu long.

[Illustration:  Fig. 36.—­Dysteria lanceolata.]

KEY TO MARINE GENERA OF CHILIFERIDAE.

Diagnostic characters:  Mouth never lies behind the middle of the body; the oesophagus is but slightly developed.  The undulating membranes are placed either on the edge of the mouth or in the oesophagus.  A peristomial depression leading to the mouth is absent or very slightly indicated.

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; Claparede & Lachmann ’58;
Buetschli ’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 (Buetschli).  The oesophagus is extremely short and hard to make