27 



Marck. They are found enveloped in sponge, and ne- 

 ver affixed to hard bodies; the valves are but slightly 

 connected together, particularly those at the bottom : 

 the exterior form is oval subconical, formed of six la- 

 teral unequal valves with an orbicular lamina, inter- 

 nally concave, (resembling a patella), forming the base; 

 the operculum with four pieces or valves: the exterior of 

 the base being conical or convex, prevents the shell from 

 standing by itself in an erect position, when detached 

 from the substance which envelopes it. 

 Acasta Montaguii Acasta glans 



Acasta sulcata. 



CREUSIA. 



LEPAS STRIATULA. Linn. 



Pennant. 4, pi. 38, f. 7. 

 This genus, as well as the preceding, was named by 

 Dr. Leach, and with the succeeding G. Pyrgoma, are 

 the only two genera known to possess four valves, and 

 the operculum to consist of two pieces only; they are 

 in general small shells affixed to madrepore or other 

 marine substances, orbicular or conically convex, com- 

 posed of four unequal valves, united, but distinctly 



