Further study may show that Dulus can be added as a fourth subfamily.
In the second subfamily, Trochodiscida, the margin is armed with solid radial spines, lying in the equatorial plane.
The genera mentioned form together the subfamily of Triopylida.
A third subfamily, Discopylida, is formed by the building of a chambered equatorial girdle around the margin of Pylodiscus.
Conophis is a member of the subfamily Xenodontinae and, as presently understood, has no known living close relatives.
That subfamily contained at that time only two genera, Dorataspis (with seven species) and Haliommatidium (with five species).
But one genus belonging to this subfamily is represented within the limits of the United States.
The second subfamily of Tympanida are the Paratympanida, in which the two horizontal rings become closed by lattice-work.
This subfamily is composed of closely related genera which are found only in the New World.
There is no vital difference between it and the subfamily Muricin.
subfamily sub·fam·i·ly (sŭb'fām'ə-lē)
A taxonomic category ranking between a family and a genus.