OTHER BIVALVE MOLLUSKS 



115 



Fig. 90 



protects the 



The great California Piddock, 

 ParapJiolas californica^ Conr., is 

 represented in Figure 90, which 

 well illustrates its main features. 

 The upper end of the shell is com- 

 posed mainly of large scales of epi- 

 dermis. Near the line of union of 

 the two valves there are accessory 

 plates, long, straight, and smooth. 

 The shells are white, rather deli- 

 cate, and are three or more inches 

 in length. The rocky dust which 

 the animal obtains in the process of 

 excavation he uses in building up 

 a strong, conical chimney, which 

 siphons. 



Martesia intercallata^ Cpr., the Shell-boring Pid- 

 dock, is a very small borer, from the southern fauna, 

 which is sometimes found in large shells like that of 

 the Haliotis. Its presence sometimes greatly dis- 

 turbs the occupant of the shell, especially if its bur- 

 row has been carried nearly through the pearly lin- 

 ing. In that case a knob is built up as a defense 

 against the intruder, and these knobs, or "blisters," 

 as they are often called, may be cut out and set as 

 jewels. Occasionally, however, the borer goes clear 

 through the shell, and then there is trouble for all 

 concerned. The valves of the Shell-boring Piddock 

 gape widely in front, and the outer entrance to its 

 burrow is quite small. 



There is a very singular and very destructive mol- 

 lusk, which lives especially in San Francisco Bay, 



