HABITS AND ECONOMY OP THE MOLLUSCA. 17 



Not mucli is known respecting the individual duration of the 

 shell-fish, though their length of life must be very variable. 

 Many of the aquatic species are annuals, fulfilling the cycle of 

 their existence in a single year ; whole races are entombed in 

 the wintry tide of mud that grows from year to year in the beds 

 of rivers, and lakes, and seas ; thus, in the Weald en clay we find 

 layer above layer of small river-snails, alternating with thin 

 strata of sediment, the index of immeasureably distant years. 

 Dredgers find that whilst the adults of some shell-fish can be 

 taken at all seasons, others can be obtained late in the autumn or 

 winter only; those caught in spring and summer being young, or 

 half-grown; and it is a common remark that dead shells (of some 

 species) can be obtained of a larger size than any that we find 

 alive, because they attain their full-growth at a season when our 

 researches are suspended. Some species require part of two 

 years for their full ' development ; the young of the doris and 

 eolis ai'c born in the summer time, in the warm shallows near 

 the shore ; on the approach of winter they retire to deeper water, 

 and in the following spring return to the tidal rocks, attain their 

 full-gi'owth early in the summer, and after spawning-time dis- 

 appear. 



The land-snails are mostly biennial ; hatched in the summer 

 and autumn, they are half-grown by the winter-time, and acquire 

 their full-growth in the following spring or summer. In confine- 

 ment, a garden-snail will live for six or eight years ; but in their 

 natural state it is probable that a great many die in their second 

 winter, for clusters of empty shells may be found, adhering to one 

 another, under ivied walls, and in other sheltered situations ; the 

 animals having perished in their hybernation. Some of the 

 spiral sea-shells live a great many years, and tell their age in a 

 veiy plain and interesting manner, by the number of fringes 

 (varices) on their whirls ; the contour of the ranella and murex 

 depends on the regular recurrence of these ornaments, which 

 occm* after the same intervals in well-fed individuals, as in their 

 less fortunate kindred. The Ammonites appear, by their varices. 



