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), which 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 Mytilidae; 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: 



o t Shell not equivalve, animal attached ........................ 1. ANOMIIDAE 



a, Valves of shell alike. 

 &! Not attached by byssus .................................... 2. ABCIDAE 



6 a Attached by byssus ..................................... 3. MYTILIDAE 



FAMILY 1. ANOMIIDAE. 



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

 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; 

 sexes separate: 4 genera. 

 Anon^a g ephi PP ium ANOMIA L. Right valve flat ; left valve convex : 



1, ivarve- 2 40 S P ecieS - 



right valve; 3, notch. A. ephippium L. (A. simplex D'Orbigny). 



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. in 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. ABCIDAE. 



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. 



