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, the right edge convex.  The ventral surface is flat, the dorsal surface is arched in the middle region of the body.  The edges being flat are somewhat more transparent than the remainder of the body.  The ventral surface is striated by longitudinal straight or slightly curved lines, the dorsal surface is smooth and without cilia. (Maupas describes bristles on the back, but this is not corroborated.) The adoral zone is fairly well developed, but not distinctly marked off from the remaining ventral surface.  It begins on the right side and extends entirely around the frontal margin and down the left side below the middle of the body, where it turns suddenly to the right, entering the slightly insunk peristome.  The mouth leads into a short, indistinct oesophagus.  One contractile vacuole is situated in the dorsal swelling at the posterior end of the animal.  Macronucleus double, one in each side of the dorsal swelling.  Movement is slow and creeping, with a peculiar method of contracting the more hyaline edge, which may turn upward or around a foreign object.

Fresh (?) and salt water.

Peritromus emmae Stein.  Fig. 49.

With the characters of the genus.

[Illustration:  Fig. 49.—­Peritromus emmae, ventral and lateral aspects.]

KEY TO THE MARINE GENERA OF OXYTRICHIDAE.