PHOLAS. 21 



in oblique rows. The cartilage which unites the valves at 

 the back is quite different from the ligament of other bi- 

 valves, so that when the animal is removed they fall apart. 

 The edges do not meet all round, but are open in front to 

 admit the passage of the food or sucker, and at the hinder 

 or upper end to give room for the tube to pass through the 

 upper part of the hole. There are no hinge-teeth, but 

 from under the place in each valve where the teeth usually 

 appear, a cui'ved flattened free rib protrudes, which is cha- 

 racteristic of nearly all the Pholaclicla. The mantle of the 

 animal is entire, excepting where the foot protrudes. A 

 remarkable circumstance in the shells of this faoiily is the 

 peculiarity and variety of their accessory valves or plates, or 

 other parts. When the animals are young, and the work of 

 excavation is going on, the shells are quite simple, and have 

 no supernumerary contrivances of the kind ; but afterwards, 

 when they are full-grown and intend to retire from active 

 life, they seem to find their retreats rather too large for 

 their comfort and safety. They find, in parts of their dwell- 

 ing, room for unpleasant currents, which might not only 

 still further increase its size, but also damage their shells 

 and disturb their repose. To avoid these evils, the various 

 species of Pholas in every part of the world set to work by 



