Synopsis FrI'Sii-Watcr Mollusca 39 



Class LAMELLIBRANCHIA. 

 Order EULAMELLIBRANCHIA. 



Suborder SUBMYTILACEA. 

 Key to the families of Subinytilacea. 



I Ligament external 

 ) 



[ Ligament internal . 5- 



[ Hinge with cardinal, anterior and posterior lateral teeth 4. 



2. ^ Hinge with cardinal teeth only CyreneUidcr. 



\ Hinge with lateral teeth only (no true cardinals) or edentate 3. 



f Gills with distinct, interlamellar septa, parallel with the gill 



3. \ filaments Unionidcr. 



i Gills either without distinct, interlamellar septa or, when 



[ present, oblique to the gill-filaments Margaritauidcu. 



{ Pallial line simple Sphccriidcc. 



[ Pallial line sinuate Cyrenide. 



f Hinge with cardinal and lateral teeth Rangiidcc. 



[ Hinge without distinct teeth Dreissenslidcc. 



Family AIARGARITANID^. 



"Diaphragm incomplete, formed by the gills; posteriorly the outer lam- 

 ina of the outer gills not connected with the mantle for a considerable dis- 

 tance ; anterior end of the inner gills separated from the palpi by a gap; 

 branchial and anal openings ill-defined, and the latter not closed above ; 

 no super-anal developed ; gills without water-tubes and with scattered inter- 

 lamellar connections, which in certain places form irregular rows or with 

 continuous septa which run obliquely forwards ; marsupium formed by all 

 four gills ; larva a small semicircular glochidium, without distinct hooks ; 

 shell elongated ; sculpture of the beak concentric ; hinge-teeth imperfect ; 

 epidermis blackish." (Ortmann.) 



Genus MARGARITANA Schumacher, 1817. 



Shell elongated, usually arcuate, rounded in front, almost lacking a pos- 

 terior ridge; beaks rather low, sculpture consisting of a few coarse, par- 

 allel ridges which follow the growth lines ; epidermis concentrically striate, 

 brownish or blackish ; hinge-teeth generally imperfect or not fully devel- 

 oped, two more or less perfect pseudocardinals in the left valve and one in 

 the right, often reduced to mere tubercles ; laterals short, usually imperfect 

 or wholly v/anting : cavity of the beaks rather shallow. 



