Page:The Geologist, volume 5.djvu/38

24 of irregularity, possibly arising from the imperfection of the geological record. There are eighteen instances of this in the series.

There is a fifth form of successional order which may be illustrated by the figures connected with the genus Alveolites, which stand thus:—Sil. 4, Dey. 4, and Carb. 2, thus giving no maximum in any one period. There are three instances of this.

The genera of the Devonian period are, as a whole, comparatively poor in species, and but few of those common to it and either the Carboniferous or Silurian, or both, have their maximum specific development during Devonian times.

The following table exhibits, generally, the prominent facts of the kind just specified.

The "Totals" in the left-hand column are the same as in Table VI. The three columns headed "Species in" show the aggregate number of species found in each period belonging to the total number of genera on the same horizontal line in the column of "Totals;" thus three hundred and eighty-six species have been found in British Silurian rocks, two hundred and twenty-three in Devonian, and five hundred and ten in Carboniferous belonging to the forty-one genera common to the three periods, and so on. The three columns headed "Species ÷ Genera" show the average number of species per genus in each period and division, and are obtained by dividing the total number of species by the total number of genera in each (fractions being omitted except when considerable); thus the averages in the case of the forty-one genera common to the three periods are 9⋅4 Silurian, 5⋅4 Devonian, and 12⋅4 Carboniferous. The total averages at the bottom of these three columns are obtained thus:—Of the ninety-seven Devonian genera, fifty-five (= 14 + 41) are found in Silurian beds, and these have yielded an aggregate of four hundred and forty-two (= 56 + 386) species, giving an average of eight per genus, and so on for the other periods. The right-hand three columns show the number of genera, which in the various divisions have their maximum specific development in each period; for example, of the forty-one genera common to the three periods, thirteen had their