CHAPTER XIII: THE ROCK-DWELLERS 



Family PETRicoLiDyE 



Shell oval, thin, white, gaping behind; hinge with two or 

 three teeth, no laterals; pallial sinus deep; epidermis thin; mantle 

 thickened and recurved over shell at edge, closed in front, except 

 for small pedal opening, foot narrow, pointed; siphons long, sep- 

 arate at least part way, tips fringed. Animal free, but frequently 

 boring into clay and soft rocks. 



Genus PETRICOLA, Lam. 



Characters of the family. 



The Pholas-shaped Rock-dweller (P. Pholadiformis, 

 Lam.) is a narrow and oblique oval, with corner beaks, rayed 

 faintly all over, wrinkled into ridges on the short anterior area. 

 The long siphons are divergent for almost their entire length. 

 The species is common on sandy or muddy beaches of New Jersey. 

 Farther south it finds a suitable hiding place in masses of coral 

 sediment. Length, 2^ inches. 



Habitat. — Atlantic coast. 



P. carditoides, Conr., adapts its form to the burrow it in- 

 habits. It often becomes very thick shelled and rough, and 

 always dingy white. Preempting a suitable cranny when young, 

 the shell grows to fit it. Length, i to 2 inches. 



Habitat. — West coast. 



Genus SAXIDOMUS, Conr. 



Shell large, oval, thick, with three strong, divergent cardinal 

 teeth in the hinge. Ligament large, external. Large edible 

 clams of the northwest coast. 



The Giant Rock-dweller (5. giganteus, Desh.) has the gen- 

 eral form of the eastern hard-shell clam, but is larger, on an 

 average. The extreme is between five and six inches in length. 



345 



