SHELLS. 35 



same manner. The fresh layers are parallel to 

 the margin of this opening, called the mouth, and 

 the meeting of the edges of the new and old 

 matter is often marked by a ridge more and less 

 elevated. 



The shell of the adult animal is often armed 

 with inequalities on the surfaces, as spines or tu- 

 bercles, which do not appear in the young shell. 

 I will now give you a summary of the lesson of 

 to-day. 



The mollusca are often placed in situations of 

 great danger. They are exposed to the dashing 

 of the waves, often borne by the violence of 

 storms and cast against rocks or stones. Some, 

 placed in rapid rivers, are occasionally hurried 

 along by the impetuosity of the stream. God, 

 however, always makes a suitable provision for 

 the circumstances under which he places his 

 creatures, and when he apportions to them spots 

 of danger, arms them with the means of protec- 

 tion and defence. Thus we find that the shell 

 and the habits of the mollusca are adapted to the 

 situations which they occupy. Some that belong 

 to the rapid rivers of America have an exceed- 

 ingly hard and substantial shell, fitted to contend 

 with the most boisterous elements ; others, on 

 the contrary, by their very lightness are enabled 

 to float on the surface of the water, and offering 

 D 2 



