Thus it is probable that some four-gilled form was the progenitor of the dibranch cephalopods.    From Wordnik.com.  [On the Genesis of Species] Reference

Now the four-gilled kinds (judging from the only existing form, the nautilus) had the auditory organ in a very inferior condition of development to what we find in the dibranch; thus we have not only evidence of the independent high development of the organ in the former, but also evidence pointing towards a certain degree of comparative rapidity in its development.    From Wordnik.com.  [On the Genesis of Species] Reference