The horizontal lines may represent successive geological formations, and all the forms beneath the uppermost line may be considered as extinct.

The three existing genera, a14, q14, p14, will form a small family; b14 and f14, a closely allied family or subfamily; and o14, i14, m14, a third family.

These three families, together with the many extinct genera on the several lines of descent diverging from the parent form (A) will form an order; for all will have inherited something in common from their ancient progenitor.