MUSSELS AND PECTENS 37 



covered with a light brown epidermis, especially near 

 the edges. This shell is sometimes found solitary, 

 but at other times in large masses, grouped around 

 the base of a seaweed, or in some similarly protected 

 situation. The common length is about an inch. 



Modiolus plicatulus^ Lam., the Plaited Horse- 

 mussel, is really an Atlantic species, being extremely 

 abundant in sheltered bays around the city of Bos- 

 ton. It doubtless came to California with seed 

 oysters, which were planted in San Francisco Bay, 

 where it may now be found in considerable numbers, 

 and where it may ultimately become as abundant as 

 it once was in the celebrated "Back Bay" of Boston, 

 a place now converted into solid ground and covered 

 with fine streets and elegant buildings. It has a 

 rather pretty shell, about two inches long, covered 

 with a glossy epidermis. It is marked with a great 

 number of small but very distinct plaits or ribs, 

 radiating forward from the umbo. The ligament is 

 long and strong, and the interior of the shell is tinted 

 with purple. 



It is ex- 

 tremely in- 

 teresting to 

 note the 



different habits which members of the same family 

 now possess, and which they may have acquired long 

 ago and passed down to posterity. The long, singu- 

 lar shell shown in Figure 18 is evidently a kind of 

 mussel, but its owner prefers a safe, sheltered retreat 

 to a mere anchoring place on the side of a wave-swept 

 cliff. The name of this species is Adula falcata^ Gld., 



