and the Elements in which they live. 379 
that the natural gradation established by their structure among 
the upper groups in the class of Gasteropoda, agrees with their 
natural connection with the elements in which they live in the 
order which I have assigned to these, the types of Gasteropoda 
which are lowest being exclusively marine; the highest, equally 
fluviatile and terrestrial; and among these the fluviatile ranking 
immediately above the marine, and the terrestrial ranking highest, 
and the proportion of the fluviatile in the whole class being still 
larger than in the class of Acephala, inasmuch as the structure 
of Gasteropoda is also a higher degree of development of Mollusca 
than that of Acephala, and the first terrestrial type in the animal 
kingdom in the gradation of its structure making its appearance 
4m the class of Gasteropoda. 
The Cephalopoda are highest among Mollusca as aclass. They 
rank so high as to rival in the complication and development 
of their structure even some of the Vertebrata, and strange to 
Say, we have among them only marine types, not a single flu- 
Viatile representative, nor a single terrestrial one. This fact 
would at first seem to be in direct contradiction with the state- 
ments made before, if it were not for the circumstance that this 
class in itself as represented in our days does not seem altogether 
‘Teduced in comparison with the other two, if we could not be 
Satisfied that its perfect period of development were the former 
geological ages when its numbers were far greater than at present, 
a circumstance which places the whole class in peculiar relations 
to its type, which must be rather appreciated under the point of 
View of the conditions which prevailed in former ages, when 
the ocean covered more extensively the whole surface of the 
globe than at present; so that the type with its high organization 
Must be considered more with reference to its development in 
former ages, than to what it 1s now, as at present the class is pro- 
Portionally reduced, and it is well known, and it will be further 
Mentioned with reference to other types, that in earlier periods 
however high animals might have ranked by their structure, they 
Were all marine, as we know fishes to have been the only repre- 
Sentatives of Vertebrata in earlier periods. 
