57 



I. PHOLADID^.. 



THE PHOLAS TRIBE. 



The first tribe of lamellibranchiate bivalves is that of 

 which the Pholas is the type. Such an arrangement does 

 not exactly imply that the Pholas and its allies are lower 

 in organisation than all other ConcMfera, but rather that 

 they are among an assemblage of tribes which seem to take 

 their place as it were at the bottom of the table, and to 

 link the Tunicata with the higher mollusks. In many re- 

 spects a Blya resembles more nearly an Ascidian, than a 

 Pholas does; but the customary arrangement is most conve- 

 nient, and not so far from the truth as to call for alteration. 



The Pholadid(E are shelled acephalous mollusks, having 

 more or less elongated bodies, produced posteriorly into a 

 long siphonal tube, divided at its extremity, the orifices 

 being cirrhated. Anteriorly the mantle is closed, except 

 where a small orifice is left for the passage of a clavate and 

 truncate, sometimes nearly obsolete, foot. Other charac- 

 ters are such as belong to the class, or are peculiar to par- 

 ticular genera. The shells are equivalve and inequilateral, 

 always more or less gaping. They have no true hinge, and 

 the ligament is almost or altogether suppressed. Beneath 

 the summit of each valve there is a curved calcareous 

 process. 



All the members of this tribe are borers into stone, clay, 

 wood, or other substances. Their habits and history are 

 described under the several genera. 



VOL. I. I 



