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have been employed, which are best explained by the following 

 diagram, Fig. 14. 



Ventran and dorsan 

 express only the posi- 

 tion of the shell, this 

 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 

 essentially based on 

 the same system. 

 Thus, for e x a m pie, 

 proximo-ventran ex- 

 presses the position of 

 the siphuncle in No- 

 thoceras and Bathmo- 

 ceras, where this organ 

 is so close to the shell 

 that its own wall is in 

 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 

 Propioventran. 

 Extracentrove ntrari 

 Cerilroventran 



Ventrocentren 



Centren 



Dorsocentren 



Intracecctrodorsan 



CerUrodors-an 



Extra centrodarsan 



Prop'iodorsan 



Subdorsan 



Fig. 14. 



