n the animal is induced to leave the house. The
only difference between this genus and _Tintinnus_ is the covering
of foreign bodies--usually sand crystals. Movement is rapid and
restless, and peculiarly vibratory, owing to the apparent awkwardness
in moving the house. Salt water.

Tintinnopsis beroidea Stein, var. plagiostoma Daday. Fig. 47.

Synonym: _Codonella beroidea_ Entz '84.

The shell is colorless, thimble-shaped, with a broadly rounded
posterior end. The body is cylindrical. The internal organs were not
observed. Membranelles 24 in number. Length 50 mu; greatest diameter
40 mu.

[Illustration: Fig. 47.--_Tintinnopsis beroidea_.]

Var. compressa Daday '87.

The posterior end of the shell is pointed, the lower third of the
shell is swollen, the upper third is uniform in diameter and without
oral inflation or depression. Nucleus not seen.

Length 70 mu; greatest diameter 48 mu.

Tintinnopsis davidoffi Daday. Fig. 48.

The shell is large, elongated, and provided with a considerable
spine. The chitin of the shell is covered with silicious particles of
diverse size. The internal structures were not observed.

Length of shell and spine 230 mu; diameter of the oral aperture 54 mu.

[Illustration: Fig. 48.--_Tintinnopsis davidoffi_.]

The variations of these species are considerable, and as the internal
structures, such as the nucleus, are essential in fixing their
systematic position, I place them as above, provisionally, and until
further observations can be made.

KEY TO FAMILIES OF HYPOTRICHIDA.

a. Peristome indistinct; cilia on Family _Peritromidae_
ventral surface uniform and not One genus, *_Peritromus_
differentiated into cirri

b. Peristome more or less indistinct; Family _Oxytrichidae_
cilia reduced to a few rows on the
ventral surface; anal and frontal
cirri present

c. Cilia entirely reduced; frontal Family _Euplotidae_
and anal cirri present or reduced;
macronucleus band-formed or spherical

d. Peristome reduced to left edge and Family _Aspidiscidae_
does not reach over the anterior One genus, *_Aspidisca_
margin

* Presence at Woods Hole indicated by asterisk.

Genus PERITROMUS Stein '62.

(Stein '62, '67; Maupas '83.)

The body is flat, colorless or tinged with yellow, and contractile.
It is elliptical in outline, with broadly rounded ends; in some
cases the left edge is slightly incurved,