24 
THE GEOLOGIST. 
of irregularity, possibly arising from the imperfection of the geolo- 
gical 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 de- 
velopment during Devonian times. 
The following table exhibits, generally, the prominent facts of the 
kind just specified. 
TABLE YII. 
GEjS~EHA. 
Totals. 
1 Species in 
Species. 
Genera. 
Maximum 
Specific 
Development in 
Sil. 
Dev. 
Car. 
Sil. 
Dev. 
Car. 
Sil. 
Dev. 
Car. 
Peculiar to Devonian 
Common to, — 
Siloi'ian and Devonian . 
Silur., Dev., and Carb. 
Devonian and Cai*b. ... 
24. 
14 
41 
18 
56 
386 
45 
30 
223 
49 
510 
271 
4 
9-4 
2 
2 
5-4 
2-7 
12-4 
15 
9 
13 
2 
2 
2 
23 
12 
Totals 
97 
442 
347 
781 
8 
3-6 
13 
22 
6 
35 
The " Totals " in the left-hand column are the same as in Table YI. 
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 sam^e horizontal line in the column of "Totals;" thus 
three hundred and eijihty-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-r- 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 -f 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 
