INTEODUCTION. 
The classification of the Xautiloidea adopted in this volume will he 
found to differ in some of its details from the systems hitherto 
employed, the more recent writings of Xoetling, Zittel, Mojsisovics, 
and Hyatt having furnished the basis of the changes introduced h 
The aiTangement of the groups described in the following pages is 
primarily zoological, secondarily stratigraphical, each genus being 
dealt with separately, from its appearance to its extinction. 
Of the systems of classification of the Cephalopoda lately proposed, 
that of Professor Hyatt (which he states was suggested to him 
by the late Professor Louis Agassiz) is of such interest and im- 
portance as to deserve more than a passing notice. Hyatt 
considers that the generic terms Ciirtoceras^ Gyroceras, Lituites^ 
Nautilus, are merely “ descriptive terms for the dijfferent stages 
in the development of an individual, and also the different 
stages in the development or evolution of the adult forms in 
time. In other words, each of these genera as now used includes 
representatives of all the different genetic series of Tetrabranchs, 
which are either young shells in the corresponding stage of growth, 
or adult shells in the corresponding stage of evolution.” He finds 
“ that genetic affinities on a large scale are best exhibited by the 
siphuncle, particularly by the funnels ^ of the septa, which are more 
invariable than any other parts of the shell.” Accordingly Pro- 
fessor Hyatt has employed this feature as a means for distinguishing 
his larger groups, which he thus defines^: — 1. Holochoanoida, 
“ having long funnels which completely close the intervals between 
the septa.” This group is subdivided into (<x) ProcJioanites, or those 
with the necks of the septa turned forwards, e. g. Bathmoceras ; 
^ See Table A, p. xxii. 
’ This term is used by Hyatt to denote the recurred ends — “ necks ” — of the 
septa. See footnote at p. 130, where it is shown that “funnels” is not a con- 
venient term to use in this sense. 
* Proc. Boston Soc. Nat. Hist. rol. xxii. 1883, p. 260. 
