212 



THE PIDDOCKS. 



Shells are frequently found with rounded knobs inside, 

 which the occupant has built up to protect himself 

 against this burglar, which in size and shape resem- 

 bles a pea. Its valves gape widely in front, and the 

 entrance to its burrow is quite small. 



Fig. i8i represents a small specimen of the most 

 common of our species of Piddocks. Its name is Pho- 

 lalidea penita^ Conr. , Fo-la-did'-e-a pen^-i-ta. While 

 it is often even smaller than the cut, it 

 sometimes grows to a much greater size. 

 I have one preserv^ed in alchohol which, 

 including the epidermal tips, is four 

 inches long. Like the other Piddocks 

 the forward part of its 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 the softer rocks along our whole 

 coast. There are several varieties, as parva^ Tr}^on, 

 which is ver}^ small. In young specimens the for- 

 ward end of the shell is not wholly closed, but gapes 

 widely like that of the Rough Piddock. 



There is a very singular and very destructive mol- 

 lusk, which lives especially m the harbor of San 

 Francisco, and which is known as the Teredo, or 

 ship-worm. Its true name is considered by ]\Ir. Dall 

 to be Xylotria pennatifera^ Blainville, Zy-lo'-tri-a 

 pen-na-tif'-e-ra. X. setacea^ Tryon, is probably 

 another name for the same species. 



Its great end in life seems to be to bore as long a 

 hole as possible; not for the reason that it desires the 

 wood for food, but simply for the fun of boring. The 

 young of this remarkable mollusk, like those of the 



