OTHER BIVALVE MOLLUSKS 



113 



Fig. 88 



that are more than twice as long. Like 

 the other Piddocks, the forward part of 

 the shell is rounded and rasp-like, 

 while the latter part is narrow and 

 smooth. 



A triangular plate covers the hinge- 

 area, and the valves end in epidermal 

 flaps or scales. It is commonly found 

 in burrows which it constructs for itself 

 in the softer rocks which occur here and there all 

 along our coast. In young specimens the forward 

 end gapes widely to allow the strong foot to press 

 against the side of the burrow and thus assist in 

 turning the shell, whose sharp points probably wear 

 into the rock, though just the method by which it 

 bores is not clearly known. As it grows older it 

 seems to realize that it has gone far enough, and 

 closes the front of the opening in the valves with a 

 wall of shell, making them appear nearly round. 



Pholadidea parva^ Tryon, the Little Piddock, is 

 a very small species, which some consider as a re- 

 duced variety of the last. It burrows into Haliotis 

 shells. 



Pholadidea ovoid ea^ Gld., the Oval Piddock, is a 

 small, oval form, resembling in shape a small Zir- 

 phd^a crispata, whose forward end is filled out with 

 curved shell. 



Pholadidea darwinii^ Sby., Darwin's Piddock, is 

 a little borer found in rocks. The front of the shell 

 is open and circular, while the latter part is pro- 

 longed into a narrow, flattened tube, shaped like a 

 duck's bill. The shell is marked with striae, and is 



