open, and a small pocket-like hollow on its left border indicates
the region of the mouth. The adoral zone runs into this pocket
and the mouth is located in its lower right-hand corner. In _U.
transfuga_ the right border is generally described as having a
membrane of extreme delicacy. I was unable to see such a membrane in
this form, but in its place there are 2 flagella-like cirri extending
from the margin of the mouth-opening into the peristome, and these
vibrate slowly. I do not believe these could be the moving edge of an
undulating membrane, for they are quite distinct. The macronucleus is
spherical instead of band-form, and a single micronucleus is closely
attached. This is unlike the European species _U. transfuga_, in
which the nucleus is elongate. The contractile vacuole lies between
the two sets of posterior cirri. There are no marginal folds like
those of the European species.

Length 40 mu; width 25 mu. Common.

[Illustration: Fig. 55.--_Uronychia setigera_.]

Genus ASPIDISCA Ehr. 1830.

(Perty '52; Cl & Lach. '58; Stein '59; Quennerstedt '65, '67, '69;
Mereschowsky '79; Kent '81; Buetschli '88.)

Small, colorless, and rigid forms, with nearly circular to oval
contour. The left side is usually straight, or at least but slightly
convex. The right side is much more convex, and the right margin is
considerably thickened. The ventral side is flat, the dorsal surface
convex, with from one to several longitudinal ridges which run more
or less parallel with the right edge. The peristome is limited to the
left edge, where it forms a small depression which may or may not
reach the anterior border, but which in no case runs around the
anterior margin. The left peristome margin in some cases grows
over the peristome depression toward the right, thus making a sort
of cover for the peristome. In the posterior region is a deep
depression, from which 5 to 12 cirri take their origin. Seven or 8
cirri are placed in the anterior half of the ventral surface and
are arranged more or less in rows. The anus is on the right side in
the region of the anal cirri (Stein). The contractile vacuole is
generally on the right side and similarly located. The macronucleus
is a horseshoe-shaped body. Movement rapid, somewhat in circles, and
rather uniform.

Fresh and salt water.

Aspidisca hexeris Quennerstedt '67. Fig. 56.

The carapace is elliptical, about 1-1/2 times as long as broad,
rounded at the extremities. The left