We may suppose that the numbered letters in italics represent genera, and the dotted lines diverging from them the species in each genus.

The diagram is much too simple, too few genera and too few species being given, but this is unimportant for us.

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.