406 Ainerican Seashells 



considered by some to be of family or subfamily rank. See Frizzell and Myra 

 Keen. 



Genus Mercenaria Schumacher 1817 



The Hard-shell Clams or Quahogs belong to this genus. The shell is 

 large and thick; lunule large, heart-shaped and bounded by an incised line. 

 Inner margin crenulate. 3 cardinals in each valve. Left middle cardinal 

 split. Formerly placed in the genus Venus many years ago, but almost 

 universally placed in a genus by itself by modern workers. 



Mercenaria mercenaria Linne Northern Quahog 



Plate 32h 



Gulf of St. Lawrence to Florida and the Gulf of Mexico. 

 Introduced to Humboldt Bay, California. 



3 to 5 inches in length, ovate-trigonal, about % as high, heavy and quite 

 thick. Moderately inflated. Sculpture of numerous, concentric lines of 

 growth or small riblets. Near the beaks these lines are prominent and dis- 

 tantly spaced. The exterior center of the valves has a characteristic smoothish 

 or glossy area. Exterior dirty-gray to whitish; interior white, commonly 

 with purple stainings. The entire lunule is % as wide as long. The form 

 notata Say from the same region is externally marked with brown, zie^ag 

 mottlings. This species is very common and is used commercially for 

 chowders and as clams-on-the-half-shell or "cherrystones." Also known as 

 the Hard-shelled Clam. Do not confuse with M. caTnpechiensis. 



Mercenaria mercenaria texana Dall is a subspecies from the northern 

 Gulf of Mexico region. It is characterized by a glossy central area on the 

 outside of the shell, but has large, irregular, coalescing, flat-topped, con- 

 centric ribs. 



Mercenaria campechiensis Gmelin Southern Quahog 



Plate 32g 



Chesapeake Bay to Florida, Texas and Cuba. 



3 to 6 inches in length, very similar to mercenaria, but much more obese, 

 a heavier shell, lacks the smooth central area on the outside of the valves, 

 and the entire lunule is usually as wide as long. Always white internally. 

 Rarely it has a purplish stain on the escutcheon and brown mottlings on the 

 side. There have been a number of forms described. In the vicinity of St. 

 Petersburg, Florida, there is a malformed race in which there is a sharp, 

 elevated ridge passing from the umbo obliquely backward toward the pallial 

 sinus on the inside of each valve. The Southern Quahog is common but 

 has not been exploited commercially to any great extent. 



