20 EOCENE MOLLUSCA. 
of that division involves the distribution of the genera forming Professor Owen’s 
extensive family Zewthide between the two groups, and the characters on which 
M. d’Orbigny has formed his families are at the least of equal importance with those 
used by Professor Owen, I have adopted the classification proposed by M. d’Orbigny, 
but with the following modification. That author has placed Beloptera and Spirulirostra 
among the Spzrulide. Now the shells of these genera present a series of siphonated 
air-chambers associated with a rostrum, and therefore bear a much closer affinity with 
Belemnite than with the shell of the recent Spirula. I have, therefore, placed them, 
as well as Belemnosis, among the Belemnitide ; and as I agree with Voltz in considering 
Belosepia to have possessed a camerated and siphoniferous shell, I have also placed 
that genus in the same family, notwithstanding the close affinity between its remains 
and the internal shell of the recent Sepia. 
