77 
YORKSHIEE FOSSIL MOLLUSCA. BY WILLIAM CASH. 
The org-anisms which in past ages inhabited salt or fresh 
water, are as one might expect, those which are preserved in the 
greatest numbers and in the best condition in the sedimentary 
rocks. The factors which appear to have favoured the preserva- 
tion of marine and fluviatile animals as fossils, in quantity and 
variety, are: — 
1. The hardness of the protective coverings or tests. 
2. Their facility for undergoing mineralisation. 
3. The quantitative development of genera, species, and 
indi^^duals. 
4. The variety of surrounding conditions, as to depth, 
horizontal distribution, &c., &c. 
5. The early appearance on the earth, and continued per- 
sistence of forms during each successive geological 
epoch. 
No sub-kingdom seems to have fulfilled these conditions 
better than the Mollusca, the hardness of their shells fitted them 
to resist a considerable amount of abrasion, and to defy the action 
of many tendencies to decay ; their chemical composition (chiefly 
carbonate of lime) lent itself readily to changes in mineralisation ; 
they abounded in the seas and lakes of the various geological 
periods, both in genera, in species, and in individuals,they flourished 
under the most varied climatal conditions, and were found at 
all depths ; they appeared early on the theatre of the life history 
of the globe, and have held their own as a class to the present 
time. No wonder that they afford to the Geologist precious and 
most useful data for a philosophical interpretation of the world's 
early history. 
Fossil Mollusca are well represented in the county of York, 
perhaps no equal area of land can be found to surpass Yorkshire 
in the variety and interest of its Geological strata, extending 
from the Silurian rocks in the north west of the county, through 
