EXPLANATION OF TERMS. 



355 



length of Bivalves is taken horizontally, or from the anterior 



to the posterior margin. 

 Ligament. A cartilage connecting the two valves of a Bivalve. 

 Linear. Like a line. 

 Linguiform. Tongue-shaped. 



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 circum- 

 ference 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. 



Lunate. Moon-shaped. 



Mammiliform. Nipple-shaped. 

 Mandibles. Jaws. 



Marine. Belonging to, or found in the sea. 

 Median. In the middle. 

 Mentum. A chin. 



Monotlialamous. Having only one chamber, in opposition to Poly- 

 thalamous : the Argonauta is an instance. 



Muscular Impressions. The indented marks in acephalous Bivalves, 

 which indicate the insertion of the muscles, by which the animal 

 is attached to its shell. Of these there are three principal sorts : 

 lateral, central, and pallial. 1. The lateral impressions indicate 

 the animal to have two adductor muscles, as in Unio, Cardium. 

 2. A central impression shows the presence of one adductor 

 muscle, which is generally in the middle of the shell, as the oyster. 

 All these shells are, however, furnished with a third, the pallial 

 or mantle impression, which is indicated by a depressed line 

 running parallel with the ventral margin. (See Adductor Muscle.) 



Nidus. A nest. 



Nucleus. The first formed part of a shell. 



Obsolete. This term implies a faint indication of particular characters. 

 Thus, very slight or partially indistinct strise upon a shell are 

 a A 2 



