46 THE SHELL. 



and Ovulum volva) ; it is represented by the slit in Scissurella, 

 the tube of Typhis, the perforation in Fissurella, and the series 

 of holes in Haliotis. 



The margin of the aperture is termed the peristome (less 

 frequently peritreme); sometimes it is continuous or entire 

 (Cyclostoma), or becomes continuous in the adult (Carocolla) ; 

 very frequently it is interrupted, the left side of the aperture 

 being formed only by the body-whorl. The right side of the 

 aperture is formed by the outer lip {labrum)^ the left side by the 

 inner or columellar lip {labium), or partly by the body-whorl 

 [termed the " wall of the aperture " (parietal ivall), by Pfeiffer]. 

 The aperture is descending, deflected, when it does not follow 

 the spiral of the shell, but turns downwards. This occurs in 

 many shells. Helix, etc. 



Sometimes it departs from contact with the preceding whorl, 

 as frequently in Cylindrella. 

 The aperture is : — 



Longitudinal, when its greatest diameter is parallel with the 

 axis of volution. 



Transverse, reverse of longitudinal. 



Oblique, greatest diameter oblique to the axis. 



Circular, rotundate, orbicular. 



Rounded, the circle slightly interrupted. 



Auriform, ear-shaped, as in Auricula. 



Ovate, egg-shaped. 



Oblong, much longer than wide, rounded above and below. 



Lunate, semilunar, semicircular. Nerita. 



Triangular. Janthina. 



Linear, narrow. Cyprsea, Conus. 



Quadrate. Solarium. 



It is patulous when dilated and compressed when diminished 

 at its entrance. 

 The outer lip is usually thin and sharp in immature shells, 

 and in some adults (e. g. Helicella and Bulimulus) ; but more 

 frequently it is thickened, or reflected; or curled inwards 

 (inflected), as in Cyprsea; or expanded, as in Strombus ; or 

 digitate, as in Pteroceras ; or fringed with spines (foliated), as 

 in Murex. The lip is emarginate, when incised or slit, as in 

 Pleurotomaria (Ixxxii, 84) ; effuse, when the basal or anterior 

 extremity is slightly produced, depressed or reflected, as in 

 Melania ; sinuous, as in the sigmoidal margin in Janthina. 

 Interiorly it may be dentate, Nerita (Ixxviii, 56) ; plicate or 

 lamellate, when the teeth become rib-like ; ringent, having 

 numerous large jjlications, nodules or teeth, Scarabus ; sulcate, 

 when grooved within ; and labiate or marginate, when callously 

 thickened near the margin. 



Sculpture or color-markings upon the shell are longitudinal 



