PELECYPODA 571 



Arca Ij. Blood clams. Shell equivalve, or nearly so, thick; umbo 

 anterio)-; loot 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; umbo 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, J\:'' ^ r, 

 with prominent umbo directed very obliquely for- y^\' "jV>'''.\ 



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



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



cum tliick and shaggy and dark brown in color : pjg g^y 



Maine to Florida, often common on the bottom in ^^-^^ P^^""^* (Gould). 



shallow water, or attached by the byssus. 



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

 obliquely f onvards as in A. pexata, w^ith 32 to 36 ribs ; color brown ; 

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

 shallow water. 



Family 3. MYTILIDAE. 



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

 end; hinge usualh' toothless; foot cylindrical and grooved, with bj'ssus; 

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

 siphon: several hundred species. 



Key to the genera of Mytilidae here described: 



Ox Animals not boring. 



6i Umbo at extreme anterior end 1. Mytilus 



62 Umbo not quite at anterior end. 

 Ci Length 2.") mm. or over. 



di Two striated areas not present 2. Modiolus 



rfj Two distinct striated areas 3. Modiolaria 



c, 12 mm. long 4. Crenella 



Oa 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, attaclied to rocks and to 

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

 Europe. 



