discovered, were at once recognised by palaeontologists as intermediate in character between those of the overlying carboniferous and underlying Silurian systems. But each fauna is not necessarily exactly intermediate, as unequal intervals of time have elapsed between consecutive formations.

It is no real objection to the truth of the statement that the fauna of each period as a whole is nearly intermediate in character between the preceding and succeeding faunas, that certain genera offer exceptions to the rule.

For instance, the species of mastodons and elephants, when arranged by Dr. Falconer in two series — in the first place according to their mutual affinities, and in the second place according to their periods of existence — do not accord in arrangement.