492 THE DEPTHS OF THE SEA. [CHAP. x. 



affecting the first one or two hundred fathoms, they 

 were enabled to survive, the deeper part of their 

 habitat having suffered but little alteration. 



Sir Charles Lyell says : " The reader should be 

 reminded that in geology we have been in the habit 

 of founding our great chronological divisions, not on 

 foraminifera and sponges, nor even on echinoderms 

 or corals, but on the remains of the most highly 

 organized beings available to us, such as mollusca. 

 ... In dealing with the mollusca, it is those of the 

 highest or most specialized organizations which afford 

 us the best characters in proportion as their vertical 

 range is the most limited. Thus the cephalopoda are 

 the most valuable, as having a more restricted range 

 in time than the gasteropoda, and these again are more 

 characteristic of the particular stratigraphical sub- 

 divisions than the lamellibranchiate bivalves, while 

 these last again are more serviceable in classification 

 than the brachiopoda, a still lower class of shell-fish, 

 which are the most enduring of all." With great 

 deference to Sir Charles Lyell, I cannot regard the 

 most highly specialized animal groups as those most 

 fitted to gauge the limits of great chronological 

 divisions, though I admit their infinite value in 

 determining the minor subdivisions. 



The culmination of such animal groups, such as 

 we find in the marvellous abundance and variety of 

 both orders of cephalopods at the end of the Jurassic 

 and the commencement of the cretaceous period, 

 undoubtedly brings into high relief, and admirably 

 illustrates to the student, the broad distinctive cha- 

 racters of the mezozoic fauna ; but speaking very 

 generally, the more highly a mollusc is specialized 



