170 THE PEA-POD SHELL. 



many small, transverse hinge-teeth. Length about 

 half an inch. 



The family of the mussels, or the Mytilidce, My- 

 til'-i-de, comes next. They have elongated, dark 

 colored shells, and most of them spin a byssns of 

 strong threads, by which they anchor themselves to a 

 place of safety. 



A few of them are borers, as is the case with Adit la 

 stylina, Cpr. , Ad'-u-la sty-li'-na. This species has a 

 small, peg-shaped shell, looking short and stunted. 

 Epidermis brown, shell half an inch to an inch long; 

 found in clay rocks. 



A du la falcata, Gld., fal-ka'-ta, Pea-pod-shell, 

 Fig- 143- 



Fig- H3 • 



Among the difficult things to explain is the fact 

 that a mollusk, with a thin and flexible shell, can 

 bore a deep hole into hard rock. That this is done, 

 however, can be proved by any one who will exam- 

 ine the work of this species. The shell is long, nar- 

 row, and slightly curved. The inside is white and 

 pearly, while the outside is covered with a dark chest- 

 nut epidermis, which has numerous transverse wrink- 

 les. 



I found the rocks of Duxbury Reef almost alive 

 with this and other borers. The deep, narrow holes 

 are curved to fit the shell, and the animal also spins 

 a byssus, by which it attaches itself to the sides of 

 the burrow. The length of the shell is two inches. 



