The Ark Shells. Chest Shells 



A. Americana, Gray, has a larger, more oblong shell, with 

 more ribs, flat and each impressed with a median line. These 

 characters distinguish the two species, which occur together 

 from North Carolina to the West Indies. 



A. incongrua, Say, as broad as long, with wide ridges on 

 the thin, unequal valves, is about two inches long and broad. 

 The right valve is smaller, its ridges crinkled, especially behind 

 the blunt umbones. North Carolina to West Indies. 



A. transversa, Say, is rhomboidal in outline broadening 

 at the posterior end. Its ribs are deeply cut, marked by fine 

 scaly striae, and crossed by a few deep, concentric lines. Epi- 

 dermis brown. Length, i^ to 2 inches. 



Habitat. — New England to Florida Keys. 



The Cross-lined Ark (A. reticulata, Gmel.) a very small spe- 

 cies with cancellated surface, is found on the beaches of southern 

 California. Specimens of other Mexican species sometimes stray 

 northward into these waters. Such visitants are rare. A. reticu- 

 lata is found, also, from Cape Hatteras to the West Indies. 



Genus PECTUNCULUS, Lam. 



Shells orbicular in outline, with hinge teeth in a semicircle; 

 ribs radiate, margins scalloped inside ; animal with large, cres- 

 cent-shaped foot, wavy-margined; mantle open, margins plain, 

 with small ocelli; lips continuous with gills. About sixty 

 living, eighty fossil species. 



The Feathered Pectunculus (P. pennaceus, Lam.) white, 

 mottled with brown, has its beaks meeting at one end of the liga- 

 ment. It has the characteristic semi-circle of fine teeth lock- 

 ing the hinge. As in all ark shells, the number of teeth 

 increases with the growth of the shell. Diameter, 2 inches, 



Habitat. — North Carolina to West Indies. 



The Giant Pectunculus (P. giganteus, Rve.) was used by 

 western Indian tribes of tropical America for personal adorn- 

 ment. Their graves contain armlets, a favourite ornament, made 

 by grinding away all but the rim of the shell. The beaks were cut 

 into pendents, rings and beads perforated for necklaces. Bits were 

 inlaid in mosaic work. In Arizona graves of prehistoric tribes are 

 found clay images of the Pectunculus. The sea shells are evi- 

 dently known, but rare, in a region far from their native sea. 



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