INTRODUCTION. 



Apertures. 



Fig. 43, Helicina, semilunar ; 44, Pirena, pyriform ; 45, Cypraea, linear; 

 46, Trochus, trigonal; 47, Cyclostoma, rounded; 48, Chilina, poste- 

 riorly contracted ; 49, Stomatia, transversely oval; 50, Murex, longitu- 

 dinally oval. 



The entire edge of the aperture described generally, is 

 named the Peritreme, but this term can only be conveniently 

 applied in cases where, in some at least of its characters, it is 

 the same all round, so that one descriptive term is applicable 

 to the whole. As, however, this is of rare occurrence, it is 

 found convenient in descriptions to separate the rim from the 

 outer lip. In a great number of instances, this is done natu- 

 rally, by a canal, or notch at the anterior or lower extremity, 

 and by the posterior union of that part which overlays the 

 body whorl with the other portion. At these two points the 

 outer and inner lips separate from each other : we therefore 

 describe the 



Canals of the Aperture. 



When there is neither notch nor canal, anteriorly or poste- 

 riorly, interrupting the edge of the aperture, it is described 

 as entire. When there is a notch or sinus at the anterior 



