EXPLANATION OF TERMS. 297 



Mollasca inhabiting fresh- water rivers are termed fluvia- 

 tile ; those of the sea are said to be marine. An internal 

 shell is one which is covered bj some part of the animal, as 

 in Aplijs'ia, the shell of which is not seen excepting by dis- 

 section. An external shell is one which covers the animal, 

 or, at least, is not covered by it. Shells are regular or irre- 

 giilar in their growth, according to the situations in which 

 they live, or their habit as to being attached or free. 



Mollasca are littoral if they live on the shore, between 

 tides; terrestrial, if their dwelling is on land; oceanic, if 

 they inhabit the deeper waters. 



Shells are margaritaceons if pearly in texture ; margari- 

 tiferous, if they produce pearls. 



With regard to many of these terms, it is difficult to say 

 why they should be used in English descriptions. Why 

 should we increase the difficulty of a study, by using forms 

 of expression which require a -glossary to explain them? 

 Why say that a shell is bicarmated, instead of saying that 

 it has two keels, or that it is fimbriated, wdien it is adorned 

 with a lace-like fringe ? Would it not be a great improve- 

 ment, if writers on scientific subjects would adopt the habit 

 of using plain English. in their descriptions, and only call 

 in the help of a dead language when the living one failed 



