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).
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.
A sagittal septum sometimes becomes developed, beginning with two polar beams, rising from the poles of the principal axis.
In Tholospira also the second girdle is complete, and the spiral growth begins from the third or sagittal girdle.
The shell is composed of all three above-mentioned elements, of a sagittal ring, a basal tripod, and a latticed cephalis.
From the base of the sagittal ring there arise in the horizontal basal plane two pairs of lateral branches or "basal apophyses."
But in the Eutympanida both rings exhibit one simple gate only, the apical and the basal rod of the sagittal ring being lost.
An internal ascending procolumna (rod c1 in the figure of Btschli) connects the basal and ventral rods of the sagittal ring.