KEY TO THE MARINE GENERA OF PLAGIOTOMIDÆ.

Diagnostic characters: The peristome is a narrow furrow which begins,
as a rule, close to the anterior end and runs backward along the
ventral side, to the mouth, which is usually placed between the
middle of the body and the posterior end. A well-developed adoral
zone stretches along the left side of the peristome, and is usually
straight.

1. Body cylindrical; size medium; Genus _Metopus_
peristome long and turns sharply
to the left at the extremity

2. No torsion in the peristome; Genus _Blepharisma_
undulating membrane is confined
to the posterior part of peristome

3. No peristomial torsion; Genus _Spirostomum_
body highly contractile;
no undulating membrane


KEY TO THE MARINE GENERA OF BURSARIDÆ.

Diagnostic characters: The body is usually short and pocket-like, but
may be elongate. The chief characteristic is the peristome, which is
not a furrow, but a broad triangular area deeply insunk and ending in
a point at the mouth. The adoral zone is usually confined to the left