MANUAL OF CONCHOLOGY, IX. 



GUIDE TO THE STUDY OF HELICES. 



INTRODUCTION. 



I. NOTES ON THE GENERAL MORPHOLOGY OF HELICES. 

 SHELL. 



In Helices the shell is always a well developed spiral, capable oi 

 containing the entire animal when retracted, It is generally wider 

 than high, and coiled loosely so that the central column is hollow or 

 umbilicate, but in some forms it is much higher than wide, and the 

 umbilicus is closed in the adult by an expansion of the lip, or the 

 whorls are coiled in close contact, forming a solid columella. 



The general contour of the shell is excessively variable in all gen- 

 era containing many species ; and as the number of main types of 

 form is limited, parallel groups or species occur in the various gen- 

 era as shown in the following table : 

 Genera. Shell globose, Shell depressed, Shell keeled. 



The list is capable of indefinite extension ; and even those minor 

 groups called " sections " often show the same series of changes in 

 form, thus: 



(vii) 



