Page:American Seashells (1954).djvu/504

406 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.