KEY TO THE MARINE GENERA OF OXYTRICHIDÆ. Diagnostic characters: The peristome is not always marked off from the frontal area. In the most primitive forms the cilia on the ventral surface are similar to those of the preceding family (_Peritromidæ_). Usually some of the anterior and some of the posterior cilia are fused into cirri, distinguished as the frontal and anal cirri, respectively. In the majority of forms all of the cilia are thus differentiated; strong marginal cirri are formed in perfect rows, and ventral cirri in imperfect rows. In addition to the adoral zone there is an undulating membrane on the right side of the peristome, and in some cases a row of cilia between the membrane and the adoral zone. These are the par-oral cilia and they form the par-oral zone. 1. The posterior end is pointed or 2 tail-like The posterior end is rounded; 5 not tail-like 2. The front end is pointed 3 The front end is rounded 4 3. Frontal and anal cirri absent; Genus _Stichotricha_ often tube-forming Eight frontal and 3 caudal cirri; Genus _Gonostomum_ not tubiculous 4. Anal cirri present; with or Genus *_Epiclintes_ without short lateral bristles Anal cirri absent; no bristles Genus _Uroleptus_ 5. With frontal cirri 6 No frontal cirri; 2 to 3 rows of Genus _Holosticha_ ventral cirri; anal cirri small 6. Right margin of peristome straight Genus _Oxytricha_ as far as the anterior end; 5 rows ventral cirri; 5 anal cirri Right margin of peristome curved 7 7. Five rows or less of ventral cirri 8 More than 5 rows of ventral cirri Genus _Urostyla_ 8. Membranelles normal; 5-10 anal Genus *_Amphisia_ cirri; no caudal cirri Membranelles normal; 5 to 10 anal Genus _Stylonychia_ cirri; 3 caudal cirri Membranelles very large and Genus _Actinotricha_ powerful; adoral zone not continued to mouth; 5 anal cirri * Presence at Woods Hole indicated by asterisk. Genus EPICLINTES Stein '62. (Stein '62, '64, '67; Mereschowsky '79; Gruber '87; Bütschli '88.)