a. The mouth terminates a long Family _Plagiotomidæ_
peristomial furrow having an
adoral zone along the entire
left edge
b. Peristomial area a broad Family _Bursaridæ_
triangular area ending in mouth
c. Peristomial depression short; Family _Stentoridæ_
limited to the anterior end; its
plane at right angles to the
long axis of body; surface of
peristome striated and ciliated;
no undulating membranes