106 MYTILID^. 



M. hesperianus of Lamarck. It attains a considerable 

 size, and I have a specimen which measures nearly five 

 inches in length. I may here observe that Mr. Barkers 

 shell, recorded in the ' British Mollusca ' as belonging 

 to this species and eight inches and a half long, is M. 

 modiolus. Var. 2. On all rocky coasts, filling crevices 

 and crowded together, so as to prevent the free deve- 

 lopment of each individual. Var. 3. Solitary and there- 

 fore larger and more expanding. Var. 4. On floating 

 buoys and sunken wrecks, often at a considerable dis- 

 tance from land. Young shells of the ordinary kind 

 often resemble the last variety in substance and markings. 

 The common mussel is found in all our upper tertiaries ; 

 but only the sublittoral variety (M. hesperianus) occurs 

 in the Coralline Crag. It is distributed throughout the 

 northern hemisphere from the polar circle to the ^geaii 

 Sea and the coast of Morocco, and it seems to thrive 

 equally on both sides of the Atlantic. The M. trossulus 

 of Gould, from Vancouver's Island, probably differs in 

 no other respect than being called a ( ' representative " 

 species. 



Although M. edulis here inhabits the coast-line only, 

 Dr. Walker is said to have dredged it in Baffin's Bay at 

 a depth of 140 fathoms. I once obtained a fresh single 

 valve in between 70 and 80 fathoms about forty miles 

 off the Shetlands ; but it had perhaps been voided by a 

 coal-fish (Gadus carbonarius) , which frequents the shore 

 in the spawning-season. The mussel is occasionally found 

 with the common periwinkle (Littorina litorea), living 

 on the shore in a stream of perfectly fresh water during the 

 recess of the tide. The common cockle (Cardium edule) 

 and My a arenaria have the same habit; and I have 

 even seen the two latter species associated with fresh- 

 water mollusca. All of these can exist for many days 



