jj INTRODUCTION. 



The natural position of the carapace and animal, when alive, is shown in the 

 woodcuts 1 and 2, at p. 16. 



M. Reuss's plan of placing the anterior end downwards is rather confusing ; more 

 especially since others follow a different plan. 



In the Plates the species are arranged, with some few exceptions, 1 in strati- 

 graphical order ; so that the general fades of each geological group is represented with 

 tolerable clearness. 



November, 1856. 



1 Especially of the Woolwich Series. 



