Its position is well seen in a sagittal section, such as Fig. 117.
One of the annular constrictions is constantly in the sagittal plane (separating the right and left halves of the shell).
A sagittal septum sometimes becomes developed, beginning with two polar beams, rising from the poles of the principal axis.
Also in this second system the transverse girdle is first developed, secondly the lateral girdle, thirdly the sagittal girdle.
The third or sagittal girdle becomes developed from the second almost in the same manner as the second from the first.
In Tholospira also the second girdle is complete, and the spiral growth begins from the third or sagittal girdle.
Therefore the gates of these two parallel rings become bisected by the sagittal ring, each divided into a right and a left half.
But in the Eutympanida both rings exhibit one simple gate only, the apical and the basal rod of the sagittal ring being lost.
The two lateral feet (right and left) are larger and more branched than the two sagittal feet (sternal and caudal foot).
The shell is composed of all three above-mentioned elements, of a sagittal ring, a basal tripod, and a latticed cephalis.