MOLLUSCA. BIVALVIA. 
381 
BIVALVIA. 
ASIPHONIDA. 
Cloak open, like the aperture of the shell, and permitting the 
water to come immediately in contact with the mouth and 
gills. The margin of the cloak has a double fringe of fi- 
laments. 
I. Valves closed hy one adductor muscle. 
A. Shell free, or adhering to other bodies by a byssus only ; 
furnished with a foot. 
a. Ligament narrow, and confined to a space under the beaks. 
Pectenid^. 
b. Shell compact. 
Pecten. 
Lima. 
Plagiostoma. 
(Pedum.) 
hb. Shell foliated. 
Gryphcea. 
(Vulsella.) 
(Placuna.) 
aa. Ligament marginal, sublinear, simple or interrupted by the 
hinge teeth or pits ; shell foliated. PERNADiE. 
Perna. 
Geruillea. 
Crenatula. 
Imceramus. 
(Malleus.) 
AA. Shell fixed or cemented to other bodies ; inequivalve. 
a. Hinge plain or destitute of teeth. OstreadiB. 
Ostrea. 
Dianchora. 
(Hinnites.) 
Anomia. 
aa. Hinge furnished with teeth. SpondyMcB. 
Spondylus. 
Plkatula. 
