15 {li) Cell dorsoventrally flattened. Longitudinal nonciliated stripe ex- 

 tending from right corner of vestibulum at least one-half cell length 

 ventrally. Vestibulum transverse. Family PLAGIOPYLIDAE. 



15 (i4) Cell circular in cross section. Vestibulum oblique, triangular, or 

 circular. No such unciliated stripe 



16 (15) Vestibulum triangular, anteroventral, all ciliary rows terminating against 

 it. Living in various invertebrates. Family BALANTIDIIDAE. 



16 {15) Vestibulum otherwise. Free-living 17 



17 {16) Vestibulum circular; ciliary rows symmetrical. Cilia in left part 

 of vestibulum in closely set radial rows independent of ciliary rows 

 on general cell surface. Family CLATHROSTOMATIDAE. 



17 {16) Vestibulum generally narrow, oblique ; ciliary rows asymmetrical. 

 Cilia in left side of vestibulum apparently the modified anterior 

 part of ciliary rows to left of vestibulum. Family COLPODIDAE. 



11 



