28 LEPAS. 



composed of six triangular portions, or valves ; these 

 are separate at top, but joined at bottom, and have their 

 intervals filled with shell of the same nature, but of a 

 different texture. The valves are joined by parallel 

 laminae, or plates, which project from the edge, and, 

 closing over each other, connect these parts together, 

 and form as it were a single shell. In very young shells 

 the cone appears on all sides smooth and even, but as 

 the animal within increases in size, it presses against the 

 walls of its habitation, and thus gradually opens the 

 sutures and enlarges the cavity. It is this separation 

 which gives to the valves their triangular appearance, 

 and which may be readily observed in the old shells 

 of the common species, so abundant on the backs of 

 oysters, &c. According to Bruguiere, the upper part of 

 the shell never increases ; it remains always the same 

 as when first formed, and the augmentation of the 

 diameter of the opening, is the result only of the suc- 

 cessive developement of the lateral edges. 



The operculum, or lid, which is seen on the top of 

 the cone, is always proportioned to the diameter of the 

 opening which it is designed to close. It increases with 

 the rest of the shell by a juxtaposition of testaceous 

 matter, which takes place on the inferior edges of the 

 pieces, or valves, of which it is composed. These 

 valves are articulated together by a cruciform suture, 

 and fixed against the interior of the shell by a circular 

 ligament which admits of sufficient motion for the ani- 

 mal to raise the lid and extend its tentacula. These ten- 

 tacula, when in the water, are continually moving in a 

 spiral direction, so as to bring the food, whatever it may 

 consist of, within the vortex produced by their motion. 



The compressed shells, or barnacles, are often found in 



