| No of Species.[1] | Thickness of Strata. | No of Species to 100 feet thickness.[1] | |
| Living | 5000 | ||
| Tertiary | 272 | 2000 | 137 |
| Cretaceous | 500 | 1100 | 45.5 |
| Oolite | 771 | 2500 | 31 |
| Saliferous and Magnesian | 118 | 2000 | 6 |
| Carboniferous | 366 | 10,000 | 3.6 |
| Silurian, &c. | 349 | 20,000 | 1.7 |
The most predominant of the recent forms of mollusca are the classes of Conchifera, Gasteropoda, and Cephalopoda; these are also the most numerous in a fossil state, for of pteropodous mollusca, a few traces only occur in the tertiary strata. If the distinct species of shelly mollusca be supposed to amount to 5000 species, the numbers belonging to each of these great classes may be stated thus, in a recent state:—
| Conchifera | 1800 |
| Gasteropoda | 3100 |
| Cephalopoda | 100 |
The same classes, in a fossil state, contain in 5000:—
| Conchifera | 2086 |
| Gasteropoda | 2230 |
| Cephalopoda | 684 |
If we analyse the classes, greater discordances appear. Thus the existing conchifera, ranked in three groups, present the following proportions in 1000:—
| Conchifera plagimyona (Latreille) | 777 |
| —————— Mesomyona (Latr.) | 194 |
| —————— Brachiopoda | 29 |
but in a fossil state the proportions are,
| Conchifera plagimyona | 483 |
| —————— Mesomyona | 338 |
| —————— Brachiopoda | 179 |
In the same way it appears, that while in existing nature the shelly gasteropoda ranked in two great
