OP CREATION. 



41 



few fathoms to the deepest abysses at which any ap- 

 pearance of life is met with. Fi n 



The annexed figure (11) represents a 

 common and characteristic silurian spe- 

 cies, of the kind afterwards most com- 

 mon ; while the figures 12, 13, serve to 

 illustrate the very remarkable internal 

 partition, separating the interior of the 

 shell into several parts in the case represented, and 

 in other instances affording very singular modifications 

 of this curious principle of structure. All the shells of 

 animals of this group have projecting plates of shell, 

 more or less prominent, passing up from the centre of 

 the larger or upper valve. (See fig. 12.) 



OR- 



Fig. 12 



Fig. 13 



PENTAMERUS. 



The Pteropoda of the silurian formations were 

 probably numerous and powerful, attaining a far 

 larger size than they have done at any subsequent 

 period. Several species of a genus (Creseis) still 

 represented by some small Mediterranean species 

 have been determined from some of silurian rocks ; 



