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 70µ; greatest diameter 48µ. 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 230µ; diameter of the oral aperture 54µ. [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 _Peritromidæ_ ventral surface uniform and not One genus, *_Peritromus_ differentiated into cirri b. Peristome more or less indistinct; Family _Oxytrichidæ_ cilia reduced to a few rows on the ventral surface; anal and frontal cirri present c. Cilia entirely reduced; frontal Family _Euplotidæ_ and anal cirri present or reduced; macronucleus band-formed or spherical d. Peristome reduced to left edge and Family _Aspidiscidæ_ 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 emmæ Stein. Fig. 49. With the characters of the genus. [Illustration: Fig. 49.--_Peritromus emmæ_, ventral and lateral aspects.]