102 



Marine Shells of the Western Coast of Florida 



apex 



an tare ' - 



body whorl- 



umbilicus 



sptre 



-posterior canal 



- columella 



- inner Up 



aperture 

 -outer lip 



anterior canal 



Fig. 6. Diagram showing the characteristic features of a gastoropod. 



The gasteropod shell consists of one unit and is called univalve 

 in distinction from the pelecypod shell which has two units and is 

 called bivalve. The univalve shell may be regarded as a cone spirally 

 twisted about its central axis or columella. Almost any possible mod- 

 ification of this cone may develop from the flat and expanded Sinum 

 to the slender elongated Terebra or the tentlike Fissurella. 



Modifications of the primary cone produce shells which vary 

 widely in the relative proportions of different parts, but all have 

 certain features in common which it is needful to know in order to 

 proceed with classification. The living mollusk bears the shell with 

 the apex posterior and the aperture downward, but for the purpose 

 of examination the univalve shell should be held with the apex up- 

 ward and the aperture facing the observer. The apex is the extreme 

 top of the shell and in many cases is the nucleus or embryonic 

 shell with which the young mollusk emerged from the egg. Below 

 the apex are several turns or whorls, separated by more or less deeply 

 impressed sulci called sutures. These sutures may be shallow or 

 deeply cut, and the whorls themselves may be convex or more or less 



