Although Quennerstedt's description of _L. pusillus_ makes no mention
of a caudal bristle, the size and other characters are so closely
similar that I hesitate to make a new species. The bristle is
extremely delicate, scarcely thicker than a cilium, and easily
overlooked, yet with proper focussing of the condenser I found it on
every specimen examined.


[Illustration: Fig. 43.--_Lembus pusillus_.]


KEY TO MARINE GENERA OF OPALINIDÆ.

Diagnostic characters: The form is oval, and the body may be short or
drawn out to resemble a worm. They are characterized mainly by the
absence of mouth and pharynx.

Anterior end not pointed; body Genus *_Anoplophrya_
cylindrical; tapering

Anterior end pointed; body elongate; Genus _Opalinopsis_
cylindrical; tapering

* Presence at Woods Hole indicated by asterisk.


Genus ANOPLOPHRYA Stein '60.

(Stein '60; Claparède '60; Leidy '77; Vejdovsky '79; Kent '81;