The coat of the inner ball is thin and papery, and opens by an apical mouth.

Apical: at, near or pertaining to the apex; usually of a wing.

A small common central rod separates the two united pectoral feet from the two other spines (the caudal foot and the apical horn).

Apical horn half as long as the shell and the three feet; all four spines straight, cylindrical, with thickened club-shaped end.

But in the Eutympanida both rings exhibit one simple gate only, the apical and the basal rod of the sagittal ring being lost.

Ring half ovate or nearly triangular, with three curved edges and three prominent corners, one apical and two basal protuberances.

Apical horn short, pyramidal, three primary feet half as long as the shell and twice as long as the three secondary feet.

Apical horn and the three divergent feet of equal size, somewhat longer than the ring, slightly curved, in the distal half forked.

Apical horn and the six divergent feet twice to three times as long as the shell, bristle-shaped, irregularly curved and branched.

The apical horn and the three parallel feet are of equal size and similar form, straight, conical, one-sixth as long as the shell.