The analogy of the tertiary to the actual system of organic nature is very apparent in these numerical proportions, and the distinctness of both from the older types in the lower strata is one of the most remarkable and important generalisations in geology.
Nearly all the fossil mollusca, even in the tertiary system, belong to extinct species, a large proportion to extinct genera, particularly among the cephalopoda, brachiopoda, and mesomyona.
The following tables[1], will exhibit the numerical proportion of species of particular genera in the living and ancient systems of nature, and illustrate other important truths.
Table I.—Genera containing many Living Species. (gasteropoda.)
| Cypræa | Conus | Voluta | Strombus | Murex | Fusus | Ceri- thium |
Mitra | Pleu- rotoma | |
| Living species | 135 | 181 | 66 | 45 | 75 | 67 | 87 | 112 | 71 |
| In Cainozoic strata | 19 | 49 | 32 | 9 | 89 | 111 | 220 | 66 | 156 |
| In Mesozoic strata | 2 | 1 | 3 | 1 | |||||
| In Palaeozoic strata |
In this table the strong analogy of the tertiary and living forms of animals, and their distinctness from those of earlier date, are very decided.
- ↑ Taken from the Guide to Geology, 3d edition.
