26 LEPAS. 



The compressed shells, or barnacles, are generally 

 composed of five valves, not articulated, but merely 

 connected together by a membrane which bounds their 

 circumference. These valves are supported on a flexible 

 pedicle, which the animal can lengthen and contract, or 

 move in any direction at pleasure. 



The conical, or acorn shells, on the contrary, are gene- 

 rally composed of six valves, which form a kind of 

 pyramidal tube, having all its parts strongly articulated, 

 or joined to each other by delicate sutures. The base is 

 solid and testaceous, by which they firmly fix themselves 

 to other bodies, There are four valves on the top of the 

 shell which form an operculum, or lid ; these valves 

 open or close, according as the animal protrudes its body, 

 or shrinks within its shell. These appendages are 

 entirely wanting in the compressed shells ; nor have they 

 any thing to correspond with them, unless their five 

 great valves may be considered as the operculum, and 

 the ligament to which they are attached, as the base 

 corresponding to that of the acorn shell. 



The essential differences, therefore, of these two divi- 

 sions of shells, or genera, as they are constituted by 

 Bruguiere, consists, — 



1st. In their support, which is testaceous in the 

 conical, and membranous in the compressed 

 shells. 

 2nd. In the structure of the shell, which is com- 

 posed of articulated pieces in one kind, while the 

 valves of the other division are connected by 

 membrane only. 

 3rd. In the operculum, with which the conical shells 

 are exclusively provided. 

 These reasons, it must be confessed, are strongly in 



