412 American Seashells 



Genus Irus Oken 1815 

 Irus lamellifera Conrad Californlan Irus Venus 



Plate 3ir 



Monterey to San Diego, California. 



I to I % inches in length, usually oblong, although some specimens may 

 be almost round. Characterized by about a dozen, strongly raised, concentric 

 lamellae or thin ridges. Shell whitish and with a chalky texture. Moderately 

 common. Found burrowing in gray shale from low water to several fathoms. 



Subfamily MERETRICINAE 

 Genus Tivela Link 1807 



Tivela floridana Rehder Florida Tivela 



Palm Beach County, Florida. 



% inch in length, subquadrate, beaks in the center, highly polished and 

 with microscopic growth lines near the margins. Exterior glossy, tan or 

 purplish. Interior mottled with purplish brown. This is the only Tivela 

 recorded from eastern United States. Uncommon offshore. 



Subgenus Pachydesma Conrad 1854 

 Tivela stultorum Mawe Pismo Clam 



Plate 3ih; figure iSd 



San Mateo County, California, to Lower California. 



3 to 6 inches in length, ovate, heavy, moderately inflated, glossy-smooth, 

 except for weak lines of growth. Ligament large and strong. Color brownish 

 cream with wide, mauve, radial bands. Bands may be absent. Posterior end 

 marked off by a single, sharp thread. Lunule lanceolate and with vertical 

 scratches. Periostracum thin and varnish-like. A common and edible species. 

 This is the only West Coast Tivela, but it has received a number of un- 

 necessary names, T. crassatelloides Stearns being one of many. 



Genus Transennella Dall 1883 



Left anterior lateral fitting into a socket in the right valve. Internal 

 margins arc obliquely grooved with numerous, microscopic lines. These 

 are parallel to the growth lines at the ventral margin of the valves. 



Transennella stimpsoni Dall Stimpson's Transennella 



Figure 83a, b 



North Carolina to southeast Florida and the Bahamas. 



