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. \Ve 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 o*f a stony shell ; and shall in my next lec- 

 ture proceed to the shelly or testaceous tribe. 



