24 INTRODUCTION. 



erroneous impression, when applied to a part of the shell which 

 has no correspondence with the mouth of the animal. The 

 word aperture is used by modern writers in a general sense, 

 including the cavity, its edge, and the canals. The cavity 

 itself is distinguished in various shells as to its shape, which 

 depends much upon the degree of modification produced by 

 the last whorl. In some cases, as in Cyclostoma, where the 

 aperture stands apart from the last whorl, the shape is round, 

 or nearly so. The Scalaria presents a good example of this. 

 In others, where the inner edge or lip, wrapping over the 

 body whorl is nearly straight, the aperture is semi-lunar, or 

 half-moon shaped : this is remarkable in the " Neritacea" of 

 Lamarck, named, on that account, " hemi-cyclostomata" by 

 De Blainville. In a great number of instances, the lower 

 part of the body whorl enters obliquely into the upper part of 

 the aperture, the result being a pyriform, or pear-shaped open- 

 ing. The aperture is described as long when it is largest in 

 the direction of the axis, and wide, in the contrary case. The 

 anterior is the part at the greatest distance from the apex, 

 and the body whorl ; the posterior, the part nearest to the apex. 

 Thus some apertures are described as posteriorly contracted 

 and anteriorly widened, or the reverse. A linear aperture is 

 one contracted in its whole length, as in Cypraea. When the 

 whorls are angulated, a trigonal aperture is the result, as in 

 many species of Trochus. Some are transversely oval, that is 

 in an opposite direction to the axis, and others longitudinally 

 oval. When the whorls are formed with two outer angles, a 

 somewhat quadrated aperture is formed. There are other 

 variations too numerous to mention. 



