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.