THE APERTURE. 283 



\Yhich it may be formed at the upper or lower parts. As to 

 its general sliape_, it is sometimes, as in Cyclostoma and Sca- 

 larla, circular or nearly so ; sometimes, as in Neritinaj where 

 the inner edge is nearly straight, it is semilunar , the outer 

 edge taking the form of a half-circle. In many instances, 

 being narrowed towards the upper and widened at the lower 

 extremity, it becomes pyriform. In Cyiyrcea it is linear ^ 

 making a groove or line along the shell : tliis is owing to 

 the thickening and inflection of the edges when fully deve- 

 loped. In Trochus it is trigonal, or three-sided; and in 

 some instances it is obliquely, longitudinally, or transversely 

 oval,* — longitudinal being in the direction of the axis, and 

 transverse the opposite. 



The edge, or peritreme, of the aperture is entire when it is 

 continuous without interruption all round ; but in the greater 

 number of cases it is interrupted on the inner side by the 

 last whorl, and in some cases by notches in the outer lip, 

 and in others by lower or upper canals. 



The canals of the aperture are formed by the contraction 

 of the upper part of the outer lip against the last whorl, in 

 such a manner as to leave a groove between ; or by the pro- 

 longation of the hollow tube at the lower end. The lower 

 canal is sometimes described as the " caudal canal." It is 



