In the second subfamily, Trochodiscida, the margin is armed with solid radial spines, lying in the equatorial plane.
A third subfamily, Discopylida, is formed by the building of a chambered equatorial girdle around the margin of Pylodiscus.
That subfamily contained at that time only two genera, Dorataspis (with seven species) and Haliommatidium (with five species).
The Nephrospyrida, the third subfamily, contains two very different genera only, each of which may represent a separate family.
The second subfamily of Tympanida are the Paratympanida, in which the two horizontal rings become closed by lattice-work.
There are twenty-five distinct species of the subfamily of grouse.
There is no vital difference between it and the subfamily Muricin.
The first subfamily is comprised of distinctively American birds.
Further study may show that Dulus can be added as a fourth subfamily.
The third genus of the subfamily (Stephanella) is only known from Japan.