482 TERMINOLOGY OF 



or external side, the aperture is said then to be effuse. 

 Ex. lanthina. 



The length of the aperture is measured in a line continued 

 from the apex of the spire to the base ; the breadth, in a line 

 drawn across from one side to the other. The shape is very 

 various, round, semilunar, oval, or linear, with many com- 

 binations which it is unnecessary to specify. 



The side of the aperture next the pillar, is the inner, or 

 columellar or left lip ; that on the other side is called the outer, 

 or right lip. The inner lip is sometimes called the labrum, 

 and the outer the labium. 



When the lips run into each other and form a continuous 

 rim around the aperture, they are said to be continuous ; 

 but when the superior edge of each terminates on the body- 

 whorl, the lips are disjunct, separate, interrupted, or confluent 

 with the body. The continuous rim, especially if it be ever- 

 ted, is often called the peritreme. 



The inner lip is often twisted near the base ; it may be con- 

 vex or flattened, smooth or granulous, armed with folds, or 

 plaits, or teeth, straight or oblique, and all these forms be- 

 come valuable to the systematist, since they are dependent on 

 corresponding structures in the animal, and lead him to a 

 certain interpretation of their meaning. The columella is 

 also frequently perforated. When very obvious and deeply 

 bored, this perforation has been generally, yet erroneously, 

 described as an umbilicus. 



The outer lip may be thin and acute, or thickened and ob- 

 tuse, smooth, or crenulate, or toothed, simple or reflected, 

 plain, or strengthened with an external rib or varix. The 

 rib or varix exists only in shells of zoophagous Mollusca. In 

 land shells, the lip is often thickened and everted on ma- 

 turity ; and in the genus Anostoma, at maturity, the last 

 whorl is turned upwards, so that the aperture appears upon 

 the same plane as the spire. Previous to adultness, "the 

 animal must crawl about, like other snails, with the spire of 

 its shell uppermost ; but as soon as it arrives at maturity, 

 and is about to form its complete aperture, it takes a reverse 

 position, and afterwards constantly carries its spire down- 

 wards." The reason for this remarkable structure has yet to 

 be ascertained in a better knowledge of the habits of the 

 Anostoma. 



In siphonostomatous shells, the interruption to the even- 

 ness of the rim 1 is always at the base or lower end. Some- 

 times both ends are effuse or emarginate, as in Cyprsea; and 

 in some other zoophagous genera there is a notch at the 

 superior angle of the outer lip. A sinus a little above 



