EXPLANATION OF TERMS. 397 



Incrassated. Any part more thickened than usual, or 

 than the surrounding surface. The inner lip of 

 Oliva and the hinge of Glycimeris are good examples 

 among the spiral and acephalous tribes. 



Incurved. The point turned inwards. 



Indented. A term of very comprehensive meaning : it 

 is used, generally, to express any thing depressed or 

 sunk beneath the surrounding surface, whether they 

 are lines, dots, irregular cavities, or tooth-like depres- 

 sions. Many shells are marked by indented striae. 

 The suture of others is also indented. 



Inequivalve. One valve smaller than the other, as in 

 Anomia and Terebratula. 



Inferior valve of adherent bivalves, is that by which the 

 shell is united to other substances. 



Inflated. See Fentricose. 



Irregular bivalves. Such as, from being influenced 

 in their growth by the substances in which they re- 

 side, do not present a uniformity of shape in the in- 

 dividuals of the same species. This variation is 

 almost confined to the perforating groups; among 

 which, however, as in Lithodomus, this deviation 

 from a uniform specific shape is not found. 



Lamellated. Shells whose substance is composed of 

 very thin plates or lamellae, which do not present a 

 solid surface, as in the pearl oysters. 



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 

 central. 



Length of shells. Spiral shells are measured from the 

 tip of the spire to the base, and therefore perpen- 

 dicularly : but the length of bivalves is taken hori- 

 zontally ; thus the Soleus are the longest shells of 

 this tribe, the length being taken from the anterior 

 to the posterior margin. 



