through the action of its powerful cirri. It is often quiet, usually mouth downward, and is held in place by adhesion of the tentacles. In this position it looks strikingly like a heliozoön. Length 35µ; greatest width 30µ. Not uncommon. The chief features by which this species is distinguished from the frequently described _M. pulex_ of Europe are the number of anterior cirri and the ring of true cilia in place of the central girdle of cirri. The European form is described with four anterior bristles; the present form has from 28 to 32. The radial cilia differ decidedly from the more powerful cirri and they are not in one plane, so that counting is difficult; they are not closely set. The presence of tentacles makes these forms of especial theoretical interest, especially in the light of the origin of _Suctoria_. [Illustration: Fig. 31.--Side and top views of _Mesodinium cinctum_.] Genus TIARINA R. S. Bergh '79. (Claparède & Lachmann '58.) Body subcylindrical, pointed posteriorly, two and one-half times as long as broad; encased in covering composed of separate pieces arranged in five girdles. The pieces bear processes which rest against neighboring pieces of the girdle. Mouth large, anus terminal near contractile vacuole. The macronucleus is simple and round. Salt water. Tiarina fusus (Cl. & Lach.) emend R. S. Bergh. Synonyms: _Coleps fusus_ Cl. & Lach. '58; Daday '86; Möbius '88, Lauterborn '94; Shevyakov '86. This form, which resembles _Coleps_ rather closely, was placed as a separate genus by R. S. Bergh. The skeletal parts consist of five zones of needles composed of an organized substance and embedded in the cortical plasm, the last zone coming to a point at the posterior end. The needles have lateral processes, which give a latticed appearance to the casing. The cilia are long, with a specialized crown of still longer ones at the oral end; they arise outside of the skeletal elements and do not pass between them, as in _Coleps_. KEY TO MARINE GENERA OF TRACHELINIDÆ. Diagnostic characters: Body bilateral, or asymmetrical by local prolongations; usually compressed or flattened laterally, the left side more convex than the right. The essential feature is the position and character of the mouth. This is either a long slit extending from the anterior end well down the ventral surface, or the posterior part only of a ventral furrow remains open as a round or elongate mouth some distance from the anterior end. The entire mouth region of the body is usually drawn out into an elongate tapering proboscis which is generally curved dorsally at the extremity. An oesophagus is short or absent altogether; when present it is supported by a stiff buccal armature. Cilia are uniform about entire body or limited to the flat right side. Food is swallowed. 1. a. Proboscis easily distinguished 2 from the main body b. Proboscis not marked off from Genus *_Loxophyllum_ main body; body flat; both surfaces striated