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 
zr 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 mesozoic fauna; but speaking very 
generally, the more highly a molltse is specialized 
