Fam. 4.] NATURAL HISTORi: SOCIETY. '87 



movement is performed by suddenly opening and closing the 

 valves. 



* Decidedly inequivalve; one valve nearly flat; covered with radiating 



lines rather than ribs 5, 



* Valves with radiating ribs. (A.) 



A. About 50 on each valve , 2. 



A. About 20 on each valve , /, 



1. Peden irradians, Lam. (Fig. 4.) Shell orbicular, with 

 about 20 elevated, rounded, radiating ribs, and numerous, con- 

 centric wrinkles ; ears large and nearly equal. Very variable in 

 color, the upper valve darker. Diameters 50-75. — ^ Whole coast. 

 Very common south. 



2. Peden Islandicus, Chez. Shell nearly orbicular, with over 

 50 small, scaly radiating ribs ; ears unequal. Diameters about 

 80, the distance across from the umbo greater. — Mass, and north. 

 More common north. Rather deep water. 



3. Peden magellanicus^ Gmel. Shell orbicular; the lower 

 valve nearly flat and not quite as large as the upper one ; ears 

 nearly equal ; surface covered with very numerous and very fine 

 radiating strise; the striae of the lower valve less distinct. Up- 

 per valve brownish ; lower one white. Diameters about 125. — 

 New Jersey and north. Not rare north. 



Family 4. MYTILID/E. The Mussels. 



Shell equivalve, oval or elongated, closed ; umbones anterior ; 

 epidermis thick and dark, often filamentose ; ligament external, 

 submarginal, very long; hinge with a few small teeth or usually 

 none ; inner shell layer more or less pearly ; pallial line simple ; 

 anterior muscular impression small and narrow, posterior large, 

 obscure. 



Animal marine or fluviatile, attached by a byssus ; mantle 

 lobes united between the siphonal openings ; foot cylindrical, 

 grooved. 



* Surface without radiating ribs, sometimes with faint radiating lines. (A.) 

 A. Umbones at the anterior end. (B.) 



B. Large, over 50 long /. Mytilus. 



B. Small, under 20 long ; lunate in form 5, Dreissensia. 



A. Umbones near but not at the anterior end 2. Modiola. 



* Surface with radiating ribs. (O.) 



O. Radiating ribs at both ends of the shell, but a central triangle smooth. 



3. Modiolaria. 



O. Radiating ribs all over the shell. (D.) 



