MANUAL OF NATURAL HISTORY. 157 



large, compressed ; shell transverse, peariy 

 within ; hinge-margin tuberculated ; ligament 

 marginal, external. Fluviatile. 



15. Family. — River-Solens (Mycetopidse). Mantle 



freely open ; siphons none ; foot long, cylin- 

 drical, produced, inflated at the end ; shell 

 thin, sub-cylindrical, gaping at both sides ; 

 beaks nearly central. Fluviatile. 



16. Family. — River-Oysters (iEtheriidse). Lobes of 



mantle entirely disunited, prolonged into 

 short siphons ; gills foliaceous ; foot large, 

 thick, oblong; shell irregular, attached; hinge 

 edentulous ; ligament in a groove of beaks. 

 Fluviatile. 



IY. ORDER — Pogonopods (Pogonopoda). 



Siphons indistinct or rudimentary ; animal at- 

 tached by a bundle of fibres arising from front of 

 base of foot. 



1. Family. — Mussels (Mytilidse). Mantle-margins 



more or less united ; siphonal tubes distinct 

 or wanting ; gills foliaceous ; foot small, nar- 

 row, byssiferous ; shell elongated, equivalve ; 

 hinge simple or sub-dentate ; ligament mar- 

 ginal, sub-internal; muscular scars unequal. 

 Marine. 



2, Family. — Fresh-water Mussels (Dreissenidse). 



Mantle closed ; branchial opening tube-like, 

 with a fringed orifice, anal sessile and plain ; 

 foot shorty byssiferous ; shell with the beaks 



