154 
LECTURE X. 
of the shell or crust varying in the different spe- 
cies, and the general colour being of a reddish or 
yellowish cast. We may observe here, that many 
different species of this genus occur very fre- 
quently in a fossil state; sometimes imbedded in 
chalk, and sometimes in flint. 
I have now passed through the chief tribes of 
the Mollusca nuda, or the Soft-bodied animals des- 
titute of a stony shell ; and shall in my next lec- 
ture proceed to the shelly or testaceous tribe. 
