CREUSIA. 



in this case^ closed by the four valves which compose the 

 principal part of the shell, of which the anterior valve 

 is the largest, and the others nearly equal. The opercu- 

 lum is bipartite, said by Leach, and in this he is followed 

 by Lamarck, to be undivided, but we have uniformly 

 found each part to be composed of two valves. 



The animals of this Genus, as well as of Pyrgoma, 

 are frequently found in great profusion, imbedded in 

 various species of Madrepores, they elongate their base 

 in order to keep their surface even with the Madrepore, 

 but it not unfrequently happens that the Madrepore in- 

 creases so rapidly as to cover and thus destroy them. 



The recent species are numerous. We have never 

 seen any fossil, nor are we aware that any are natives of 

 our own coasts. 



Fig, 1. Creusia gregarea. Nob. in a Madrepore. 



2. A perpendicular section of the same magnified. 



3. Outside of tlie four valves, not separated from each other, 



4. Inside of the same. 



5. Outside of the two valves of one part of the operculum. 



6. Inside of the same. 



