402 NEW YORK STATE MUSEUM 



Terminology 



In employing Hyatt's system in this group it is necessary also to 

 adopt his terminology with some alterations and additions proposed 

 by Holm and others. The less common of these technical terms are 

 here briefly defined. 



The word conch is used for the entire shell ; this may have a 

 nepionic bnlb (or preseptal cone) which is a conical part at the 

 apical end, formed before septation took place and appears as an 

 apical dilation of the siphuncle. The septate part is termed the 

 phragmocone. It consists of cameras or chambers and the siphuncle. 



The phragmocone may be an orthoceracone, which is " the older 

 stage of a straight form, and is nearly or quite straight on both 

 venter and dorsum " {see below) ; or a cyrtoceracone, which is a 

 shell " curved like Crytoceras on both venter and dorsum " ; or a 

 gyroceracone, curved in a loose spiral-like Gyroceras ; or a nautili- 

 cone, which is a closely coiled shell having an impressed zone. The 

 impressed zone is the longitudinal impression formed in the dorsum 

 by the contact of the whorls. A persistent dorsal furroiv occurs in 

 the free senile whorls of some shells, and is a remnant of the 

 impressed zone. 



The venter, ventral side or abdomen is the side on which the 

 hyponomic sinus or z'entral sintis is situated. The latter is a single 

 median bend in the apertural margin for the " ambulatory funnel " 

 or hyponome of the animal. It locates the ventral side (and not 

 the siphuncle as usually assumed). Its former position can be 

 recognized by the corresponding bend in the growth lines of the 

 conch. To avoid using, in cases where the position of the hypo- 

 nomic sinus is not known, the terms ventral and dorsal in their old 

 conception as determined by the position of the siphuncle, we will 

 in such cases denote the sides by the self-explanatory terms 

 siphonal and antisiphonal. Exogastric shells are those which have 

 the ventral sinus on the arched, external side (the great majority 

 of the forms) ; endogastric shells, those which have it on the con- 

 cave, internal side. The opposite side of the venter is the dorsum 

 or dorsal side (mostly the inner side). 



The longitudinal areas of the wj^Liv^ are termed zones {dorsal, 

 ventral, lateral zones) ; when they become flat they are called faces. 

 The junctions of the lateral faces and the abdomen are termed 

 abdominal angles and those of the lateral faces and the inner faces 

 umbilical shoulders. The lines of involutioti are the outer bound- 

 aries of the impressed zone. 



