104 WEST AMERICAN SHELLS 
The great California Piddock, 
Parapholas californica, Conr., is 
represented in Figure 92, which 
well illustrates its main features. 
The upper end of the shell is 
mainly composed of large scales 
of epidermis. Near the line of 
union of the two valves there are 
accessory plates, long, straight, 
and smooth. The shells are white, 
rather delicate, 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 protects the siphons. 
Martésia 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 
disturbs the occupant of the shell, especially if 
its burrow has been carried nearly through the 
pearly lining, in which case a knob is built up as 
a defence against the intruder. The valves of the 
little borer gape widely in front, and the entrance 
to its burrow is quite small. 
There is a very singular and very destructive 
mollusk, which lives especially in San Francisco 
Bay, and which is named Xylotria setacea, Tryon, 
the Teredo, or Ship-worm, (X. pennatifera). Its 
great end in life seems to be to bore as long a hole 
as possible; not for the reason that it desires the 
