PELECYPODA 589 



G. tumida Verr. (Fig. 927). Shell with rounded ends, compressed 

 posterially; ventral margin variable, being convex, straight, or concave; 

 color white with purple umbo ; length 4 mm. ; height 2.5 mm. ; width 1.5 

 mm.: Long Island Sound to Cape Hatteras. 



FAMILY 2. SEMELIDAE. 



Shell thin, round or ovate, gaping and unusually twisted at the 

 hinder end; external ligament short; internal ligament in a pit; hinge 

 teeth weak: many species. 



CTJMINGIA Sowerby. Shell oval, often irregular in form; hinge with 

 a small anterior cardinal and usually 2 elongated lateral teeth in each 

 valve: 10 species. 



C. tellinoides (Conrad) (Fig. 928). Shell thin and fragile, 15 mm. 

 long, 11 mm. high, 5 mm. wide, rounded in front, and triangular and 

 pointed behind; lateral teeth wanting in the left valve; color bluish- 

 white: Cape Cod to west coast of Florida; West Indies; often common 

 in shallow water. 



FAMILY 3. DONACIDAE. 



Shell more or less triangular, wedge-shaped, usually thick; liga- 

 ment external and short; foot very long; mantle open below; gills very 

 different in size : about 6 species, on the southern Atlantic coast ; marine 

 or in brackish water. 



DONAX L. Shell rounded in front and behind; mantle fringed; 2 

 cardinals and 1 lateral in each valve: 100 species. 



D. fossor Say. Shell small, elongated in front, obliquely rounded 

 and very short behind; surface with radiating striations; inner margin 

 crenulated; length 12 mm.; height 7 mm.; width 4 mm.; color white: 

 south shore of Long Island to Texas, in shallow water; common. 



FAMILY 4. MACTBIDAE. 



Shell equivalve, more or less tri- 

 angular, closed or slightly gaping; 

 pallial sinus quite short; 2 liga- 

 ments, an external and an internal, 

 the latter contained in a deep trian- 

 gular pit ; hinge with 2 divergent car- 

 dinal and usually 2 lateral teeth; 

 mantle open below; siphons long Fig. 929-3f oof ra soK^aima (Gould), 

 and fused, and with fringed orifices: numerous species. 



1. MACTRA L. Shell slightly gaping at the ends; umbo prominent; 

 foot large : 150 species, in all seas, especially in the tropics. 



M. (Spisula Adams) solidissima Dillwyn. Giant clam (Fig. 929). 

 Shell large and solid; color brown or white; length 15 cm.; height 12 



