572 



MOLLUSCA 



2. Modiolus Lamarck. Horse mussels. Shell wedge-sliaped, in- 

 flated in front; umbo not quite at front end: 70 species; cosmopolitan; 



the animals burrow in the 

 sand and gravel, and differ in 

 this respect from Mytilus. 



M. modiolus (L.) (Fig. 

 809) . Periostracum thick, 

 coarse in texture, dark brown 

 in color; interior pearly; 

 length up to 15 cm., but usu- 

 ally under 4 cm. ; usual height 

 2 cm. ; width 45 mm. : circum- 

 polar, south to New Jersey 

 and San Francisco; Europe; 

 common in deep water, among 

 rocks and gravel. 

 M. demissns Dillwyn. Shell elongate; surface marked by numerous 

 impressed radiating ribs; color greenish-yellow; pearly within, tinted 

 with purple ; length 75 mm. ; height 30 mm. ; width 23 mm. : Georgia to 

 Nova Scotia; very common on mud flats; also at San Francisco. 



3. MoDiOLARiA Gray. Shell elliptical, front end blunt; surface 

 with two areas of fine radial striations, one anterior and one posterior, 

 between which there are no striations ; cloacal siphon long : many species. 



Fig. 898 — Mytilus edulis (Lang). 1, foot; 2 

 umbo ; 3, cloacal siphon ; 4, mantle ; 5, bjssus. 



Fig. 899 Fig. 900 



Pig. 899 — Modiola modiolus (Verrill). Fig. 900 — Modiolaria nigra (Verrill). 



M. nigra (Gray) (Fig. 90^^ Shell 5 cm. long, 28 mm. high, and 15 

 mm. wide; brown in color; (interior striated area small in extent: cir- 

 cumpolar, south to Long Island, in shallow water; Europe. 



M. discors (L.). Shell oval, 25 mm. long, 16 mm. high, 10 mm. 

 wide, dark greenish in color; anterior area with 16 striations: circum- 

 polar, south to Long Island and Puget Sound, from low water to 500 

 fathoms; common north of Cape Cod; Europe. 



4. Crenella Brown. Shell small, oval or rhomboidal, with numer- 

 ous radiating striations or ribs; hinge with a tooth in each valve; foot 

 with a disc at end; byssus a single thread; umbo near anterior end: 

 about 5 species on the Atlantic coast. 



