430 
have been employed, which are best explained by the following 
diagram, Fig. 14. . 
Ventran and dorsan 
. express only the posi- 
Propioventran. tion of the shell, this 
Extracentroventran alone being superfi- 
cial. By using these 
terms and others, a 
Intracentroventran considerable degree of 
accuracy can be ob- 
tained, and others can 
also be employed if 
Dorsocentren essentially based on 
the same system. 
Thus, for example, 
Centrodorsan proximo-ventran  ex- 
presses the position of 
the siphuncle in No- 
thoceras and Bathmo- 
Subdorsan ceras, where this organ 
is so close to the shell 
that its own wall is in 
Bigs part absent and in 
part much modified. Subventran is applicable only to those forms 
in which the wall of the siphuncle is not altered or modified 
through contact with the shell, although it may lie quite closely 
against it. The other terms sufficiently explain themselves, except 
the use of ‘‘extra’’ and ‘‘intra.’’ It is not meant to confine these 
terms to the two where it is used in the diagram. It is obvious 
that these prefixes may be employed wherever they are needed. 
Thus one can say ‘‘intraventrocentren’’ for a location between 
centren and ventrocentren positions. 
It would not be proper, however, to use these prefixes on either 
side of centren for the reason that the comparisons are all made 
from the centre towards the dorsum, on the one hand, and towards 
the venter on the other. Thus everything on the dorsal side of this 
point is not inside of the centre but dorsad of this point or axis, 
and everything on the ventral side is not outside of the same axis 
but ventrad of it. I have also used Wilder’s term, mesal, for the 
plane of the siphon, instead of median. 
Subventran 
Centroventran 
Ventrocentren 
Centren 
Intracentrodorsan 
Extracentrodorsan 
Propiodorsan 
