basal rod n.
But in the Eutympanida both rings exhibit one simple gate only, the apical and the basal rod of the sagittal ring being lost.
The distance of the shell-wall from the enclosed ring is usually least on the basal rod, and greatest on the dorsal rod.
These forms may be derived from Eucoronis by loss of the basal rod of the primary sagittal ring.
The former are one upper bow or apical rod, and one lower bow or basal rod.