divisions, according to their principal food, give the following proportions in 1000:—
| Herbivorous gasteropoda | 451 |
| Zoophagous ────────── | 549 |
these divisions, in the fossil state, yield:—
| Herbivorous gasteropoda | 511 |
| Zoophagous ────────── | 489 |
It appears then, that the fossil world of mollusca differs remarkably from the actual creation in the greater proportionate abundance of cephalopoda, herbivorous gasteropoda and brachiopodous and mesomyonous conchifera. If the whole number of species of shelly mollusca of the three classes named, were supposed 1000 in the fossil and recent states, the proportions of the several groups would be nearly as under:—
| Fossils | Recent. | |
| Conchifera plagimyona | 205 | 280 |
| ───────── mesomyona | 142 | 70 |
| ───────── brachiopoda | 75 | 10 |
| Gasteropoda phytophaga | 225 | 280 |
| ─────────── zoophaga | 215 | 340 |
| Cephalopoda | 138 | 20 |
These differences, however, are by no means equal in all the several systems of strata: they are least in the newest, and greatest in the older classes of rocks. If the number of shelly mollusca in each of the three great periods be 1000, the proportionate number of the several classes may be seen in the following table, and compared with the recent creation.
| First or palæzoic Period. | Second or Mesozoic Period. | Third or Cainozic Period. | Recent | |
| Conchifera plagimyona | 150 | 228 | 268 | 280 |
| ──────── mesomyona | 102 | 202 | 70 | 70 |
| ──────── brachipoda | 320 | 105 | 8 | 10 |
| Gasteropoda phytophaga | 198 | 127 | 172 | 280 |
| ────────── zoophaga | 24 | 19 | 388 | 340 |
| Cephalopoda | 206 | 319 | 94 | 20 |
