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. 181 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 preserved 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, Tryon, 

 which is very 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 in the harbor of San 

 Francisco, and which is known as the Teredo, or 

 ship-worm. Its true name is considered by Mr. Dall 

 to be Xylotria pennatifera, Blainville, Zy-lo r -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 



