EXPLANATION OF TERMS. 297 



Iridescent. Pearly, or having the appearance of pearl. 

 Irregular. In Bivalves, when the individuals of a species 

 are not all of the same shape. 



Lanceolate. Shaped like the head of a lance. 



Lateral. Any thing on the sides of a shell. Thus the 

 lateral teeth of Bivalves, where they exist, are on one 

 or both sides of the cardinal teeth, which are always 

 in the centre, under the bosses. 



Length of Shells. Spiral shells are measured from the 

 apex, or tip of the spire to the base, and therefore 

 perpendicularly ; but the length of Bivalves is taken 

 horizontally, or from the anterior to the posterior 

 margin. 



Ligament. A cartilage connecting the two valves of a 

 Bivalve. 



Lips. The two sides of the aperture of spiral shells are 

 termed the lips. That which joins, and generally 

 folds over the lower part of the columella is called 

 the inner lip, while that part of the circumference 

 opposite is called the outer lip. 



Litoral. Belonging to, or found on the shore. 



Lobed. Having broad finger-like divisions. 



Longitudinal. In Univalves, stripes or marks from the 

 apex to the base are called longitudinal, in opposition 

 to transverse, which is applied to such as cross the 

 shell, or are parallel to the whorls : in Bivalves, 

 stripes from the anterior to the posterior margin are 

 transverse; from the bosses to the ventral margin 

 longitudinal, or more properly radiated. 



Marine. Belonging to, or found in the sea. 

 Monothalamous. Having only one chamber, in oppo- 

 sition to Polythalamous : the Argonauta is an instance. 



