570 MOLLUSCA 



Order 2. FILIBRANCHIATA. 



Gills consisting on each side of an axis, to which are attached 2 

 rows of very long, filamentous leaflets (Fig. 890, B), w'hich hang down 

 far into the mantle cavity, each row being recurved and bent upwards, 

 so that each leaflet has a descending and an ascending limb, which are 

 not joined by interlamellar partitions, except in the Mytilidde; genital 

 and kidney pores separate (except in the Anomiidae) ; foot usually weak, 

 with well developed byssus; 2 adductor muscles; mantle open below; 

 siphons absent or little developed: 4 families, all marine. 



Key to the families of Filibranchiata here described: 



tti Shell not equivalve, animal attached 1. Anomiidae 



a-i Vahes of shell alike. 



6i Not attached by byssus 2. Arcidae 



6a Attached by byssus 3. Mytilidae 



Family 1. ANOMIIDAE. 



Shell thin, with asymmetrical valves, the right valve being undennost 



and provided with a deep notch or a hole, through which a calcified 



byssus projects, fastening the animal to a rock or 



a shell; oral palps wanting; posterior adductor the 



larger ; mantle fringed, with short cirri ; no siphons ; 



foot cylindrical, expanded and grooved at the end; 



i ^ sexes separate: 4 genera. 



Fig. 896 Anomia L. Right valve flat ; left valve convex : 



Anomia ephippium ® ' 



(Verriii). 40 species. 



1, left valve ; 2. ^ 



right valve; 3, notch. A. ephippium L. {A. simplex B^Orhigny). 



Jingle shells (Fig. 896). Shell variable in shape, 

 circular or oval in outline, about 25 mm. in diameter, or larger; outer 

 surface scaly and dark colored, but in dead shells often worn off, ex- 

 posing the glistening greenish or golden mother of pearl: Nova Scotia 

 to Texas; West Indies; in shallow water/ abundant on oyster beds; 

 Europe, where it is used for food. 



A. aculeata Gmelin. Shell 12 mm. iA diameter, covered with scaly 

 or prickly radiating lines on upper valve: Long Island to the Arctic 

 Ocean; Europe; in shallow water; common north of Cape Cod. 



Family 2. AUCIDAE. 



Shell oval, symmetrical, with a heavy periostracum, and usually 

 with the radial corrugations; hinge with a row of similar comb-like teeth; 

 mantle open below; foot large, bent, and grooved; both adductors large; 

 no siphons: several hundred species; cosmopolitan. 



