INTRODUCTION. 27 



Lips, or edges of the Aperture. 



The part of the edge of the aperture next to the body whorl 

 is named the inner, or columellar lip. Posteriorly it com- 

 mences at the point of union with the outer lip, where that 

 touches the body whorl, the junction being generally marked 

 by an angle, and sometimes by a canal. Anteriorly it 

 terminates where there is generally seen a notch or canal, 

 or sudden angle, from which the outer lip proceeds. The part 

 which setting out from the body whorl, and proceeds outwards 

 at a distance from the axis, till it reaches the anterior canal or 

 notch (or its place in case of absence) is named the outer lip. 

 In many cases the edges are united in such a manner, that it 

 is difficult to distinguish where the inner lip terminates, and 

 the outer lip commences : when this is the case, it is usual to 

 describe the margin or peritr^me, as a whole, without distin- 

 guishing the parts. The outer lip, sometimes called the right 

 lip, or labrum of continental writers, is sometimes acute, not 

 being of thicker substance than the remainder of the shelL 

 In other cases it is obtuse, or thickened and rounded at the 

 edge. When thickened and turned backwards it is described 

 as reflected ; when, on the other hand, it is turned inwards 

 towards the axis, as in the Cypraedae, it is inflected, or invo- 

 lute. When it is toothed, a distinction must be observed as 

 to whether the dentations are external or internal. If the 

 teeth are small and numerous, it is denticulated; if larger, it 

 is dentated ; when expanded into a kind of wing, as in some 

 species of Strombus and Rostellaria, it is described as alated ; 

 and a family in Lamarck's system is named " Alatae," from 

 this very circumstance. In some of those which are expanded, 

 the expansion is divided into separate, attenuated portions, 

 they are then said to be digitated. 



