600 MOLLUSCA 



Suborder 7. ANATINACEA. 



Shell thin, white and pearly; hinge with or without teeth; ligament 

 external; mantles united, except Avhere the foot protrudes; 2 adductor 

 muscles; pallial line variable; siphons mostly united: 11 families, all 

 marine, including many degenerate forms, which may lack certain organs, 

 as the foot, adductor muscles, labial palps, and gills. 



Key to the families of Anatinacea here described: 



o. No spatulate hinge lamella. 



6i Pallial sinus slight or absent 1. Pandobidae 



h, Pallial sinus angulated 2. Lyonsiidae 



Oj Hinge lamella in each valve 3. Anatinidae 



Family 1. PANDOEIDAE. 



Shell free or fixed, inequivalve, pearly inside, semi-lunar or trian- 

 gular in shape ; ligament often with a calcareous ossicle ; pallial line with 

 or without a small sinus ; siphons short, united except at the end ; mantle 

 margins united : about 5 species on the Atlantic coast. 



Clidiophora Carpenter. Shell thin; valves closed, attenuated be- 

 hind; right valve flat, with a diverging ridge; left valve somewhat con- 

 vex, with 2 diverging grooves at the hinge ; a slight pallial sinus : numer- 

 ous species. 

 t 



Fig. 948 Fig. 949 



Fig. 948 — Clidiophora iriliniata (Gould). 1, siphons; 2, foot. 

 Fig. 949 — Lyonsia liyalina (Gould). 



C. trilineata (Say) (C. gouldiana Dall) (Fig. 948). Shell rounded 

 in front, hinder end curved upwards, making the dorsal margin straight 

 or concave; length 32 mm.; height 17 mm.; width 5 mm.: Nova Scotia to 

 Texas, in shallow water; common. 



Family 2. LYONSIIDAE. 



Shell thin, slightly inequivalve, the left valve being the larger; 

 pallial sinus obscure; hinge teeth absent; hinge with a narrow ledge 

 within each valve, to which adheres an ossicle; siphons very short: 

 several genera. 



Lyonsia Turton. With the characters of the family: 18 species. 



L. hyalina (Conrad) (Fig. 949). Shell ovate, pearly, and transpar- 

 ent; anterior end rounded; posterior end elongated; dorsal line nearly 



