43 



LEPAS. 



Section A. Sessile. (See Plate III. Fig. 2.) 

 B. Peduncled. (Fig. 3.) 



Shell multivalve ; affixed by the base; valves 

 unequal, erect. 



In the first division of this genus, the species 

 have mostly 6 valves, are conical, adhere by their 

 base to rocks, shells, &.c. In the second they are 

 compressed, varying in number, supported by a pe- 

 duncle or hollow membranaceous stalk. The for- 

 mer have also an operculum composed of 4 or 6 

 triangular valves closing internally the vertex of the 

 shell, and affixed by an elastic cartilage to the in- 

 terior of the upper margin. The cellular structure 

 of these shells is w^orthy of observation : the hol- 

 low striae and cavities of the triangular valves are 

 vertical, and the space between them is filled with 

 nearly horizontal filaments, forming a more com- 

 pact and even substance. The two divisions of 

 this genus, though appearing, at first sight, ex- 

 tremely dissimilar, are yet congeners in their prin- 



