24 



THE aEOLOGIST. 



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, Dev. 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 VII. 



Geneea. 



Totals. 



Species 



in 



Species. 

 Genera, 



Maximum 

 Specific 

 Development in 







Sil. 



Dev. 



Car. 



Sil, 



Dev. 



Car. 



Sil. IDev. 



Car. 



Peculiar to Devonian 



Common to, — 



Silurian and Devonian . 



Silur., Dev., and Carb. 



Devonian and Carb. ... 



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. 

 Tlie 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 

 " SpeciesH- 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. Tlie total averages 

 at tlio bottom of tliese 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-liand 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 comuion to the three periods, thirteen had their 



