Only the face or head is used for decoration or ornamentation in these pottery pieces.
1 (p. 136):
The face decorated on the rim of this pottery looks similar to the heads of clay figurines. A snake can be seen on top of the head, and another snake is decorated on the back of the pottery.
2 (p. 137):
Unlike the pottery with elaborate decorations shown so far, this piece gives an impression of simplicity, like a rough sketch. Toward the end of the Jomon period, pottery decorated with just a simple face started to appear.