592 MOLLUSCA 



FAMILY 2. PETBICOLIDAE. 



Shell elongate, or more or less irregular in form, thin, and with ends 

 rounded; 2 or 3 delicate teeth in each valve; no laterals; pallial sinus 

 deep ; mantle closed in front with an opening for the small foot ; siphons 

 long, united at the base: 2 genera, in shallow water and between tides, 

 where the animals bore into clay, limestone, and coral, making round 

 holes the diameter of the shell. 



PETRICOLA Lamarck. With the characters of the family: 13 

 species. 



P. pholadiformis Lam. (Fig. 933). Shell elliptical, swollen, gaping; 

 umbo near anterior end ; surface curved, with radiat- 

 ing grooves and ridges, those at the anterior end 

 being coarse, those back of the umbo being fine; 

 length 32 mm.; height 17 mm.; width 15 mm.; in- 

 terior with radiating furrows: Gulf of St. Lawrence 

 (Gould). to Texas, common south of Cape Cod, boring in clay. 



SUBORDER 4. CARDIACEA. 



Shell equivalve, solid, with or without hinge teeth ; ligament external ; 

 1 or 2 adductor muscles; siphons short; mantle closed below, with an 

 opening for the foot ; with or without pallial sinus : 8 families, all in salt 

 or brackish water, to one of which belongs the gigantic Tridacna gigas of 

 the Pacific, the shell of which may weigh over 200 kilograms and 

 measure a meter or more in length. 



FAMILY CABDIIDAE.* 



Shell heart-shaped, usually with radiating ridges or bands, more or 

 less chalky in texture; ligament short and prominent; pallial sinus small 

 or absent; a small and a large cardinal in each valve; laterals not con- 

 stant but usually present; inner border of shell denticulate; 2 adductor 

 muscles; mantle open below; siphons short; foot very large and bent: 

 200 species; cosmopolitan. 



Key to the genera of Cardiidae here described: 



a-! Shell with radial ribs 1. CARDIUM 



a. Shell without ribs. 



&! Shell small ; teeth large 2. L^EVICABDIUM 



b a Shell large ; teeth small or wanting 3. SEBEIPES 



1. CARDITTM L. Cockle. Shell with prominent ribs and thick, often 

 with spines or scales, closed or gaping behind; no pallial sinus; umbo 



* See ''Synopsis of the Family Cardiidae and of the North American Species," 

 by W. H. Dall, Proc. U. S. Nat. Mus., Vol. 23, p. 381, 1900. 



