PELECYPODA 571 



ARCA L. Blood clams. Shell equivalve, or nearly so, thick; umbo 

 anterior; foot pointed, with a byssus; edges of mantle with numerous 

 compound eyes: 140 species. 



A. noae L. Noah's arch. Shell elongated, wide, angular, almost 

 quadrangular; uinbo prominent, the two being far apart; valves gaping 

 below, with radial ribs; length 10 cm.: Cape Hatteras, southwards, in 

 shallow water ; West Indies ; Pacific coast ; Medi- 

 terranean Sea. 



A. pexata Say (Fig. 897). Shell thick, oblong, 

 with prominent umbo directed very obliquely for- 

 wards, and with about 32 to 36 radiating ribs, 56 

 mm. long, 53 mm. high and 37 mm. wide ; periostra- 

 cum thick and shaggy and dark brown in color: Fig 897 



Maine to Florida, often common on the bottom in Arca P exa ' ta < Gould >- 

 shallow water, or attached by the byssus. 



A. transversa Say. Shell rhomboidal, the umbo not directed so 

 obliquely forwards as in A. pexata, with 32 to 36 ribs; color brown; 

 length 37 mm.; height 25 mm.; width 37 mm.: Cape Cod to Florida, in 

 shallow water. 



FAMILY 3. MYTILIDAE. 



Mussels. Shell elongate, equivalve, with umbo at or near anterior 

 end; hinge usually toothless; foot cylindrical and grooved, with byssus; 

 anterior adductor small, posterior large ; distinct cloaeal but no branchial 

 siphon: several hundred species. 



Key to the genera of Mytilidae here described: 



o t Animals not boring. 



& x Umbo at extreme anterior end 1. MYTILUS 



6, Umbo not quite at anterior end. 

 c t Length 25 mm. or over. 



dt Two striated areas not present 2. MODIOLUS 



d 3 Two distinct striated areas 3. MODIOLARIA 



c a 12 mm. long 4. CRENELLA 



a. Boring animals 5. LITHOPHAGUS 



1. MYTILUS L. Shell wedge-shaped, being pointed in front and 

 round behind, smooth; umbo at anterior end: 65 species; cosmo- 

 politan. 



M. edulis L. (Fig. 898). Edible mussel. Color of periostracum 

 black or dark brown, within pearly with violet margins; length 10 cm.; 

 height 35 mm.; width 25 mm.: circumpolar, south to North Carolina 

 and San Francisco; Europe; very common, attached to rocks and to 

 each other; between tide lines and in shallow water; used for food in 

 Europe. 



