142 MOLLUSCA. 



CLASS IV.— LAMELLLBRANCHIATA. 



These ordinary, leaf-gilled bivalves are generally free and locomotive; — 

 a few, as the Oyster, are fixed. They are mostly equivalve (the ex- 

 ceptions occurring in the stationary species), and always unequilateral. 

 Each valve is a cone — of every grade from a flat plate to a spiral — , and 

 the valves are articulated by teeth and a ligament, and also attached to 

 each other by one or two muscles. The animal lives by filtering water 

 through its gills, which are distinct from the mantle. Nearly all have 

 a muscular foot developed from the ventral surface The Lamellibranchs, 

 according to Agassiz, differ from the Brachiopods only in " secondary 

 modifications." They do not readily group themselves into natural 

 orders : the similarity of type is great, and the points of difference are 

 not constant. The Veneridse are the typical and most highly organized 

 Lamellibranchs. 



These bivalves, though less numerous now specifically, are far more 

 abundant individually, than the Gastropods. They are all marine, 

 excepting a few widely dispersed fresh-water genera (10 out of 90), and 

 are found on every coast and in every climate, and from low water to 

 the depth of 200 fathoms. The fossil forms constitute a third part of 

 fossil shells. The genera are seven times more numerous in the newer 

 Tertiary than in the Cambrian ; while there are 800 species in the Mio- 

 eene, and only 100 in the entire Silurian series. Of 49 from the Lower 

 Silurian of New York, 26 are restricted to the Trenton Limestone. 



No. 646. Ostrea Marshii, Sow. 



This and the eight following ferms belong to the 

 family Ostreidce, being unequivalved and unequilateral, 

 and having one muscular impression. From the 

 Lias, Geislingen, Wirtemberg, and now in the Ward 

 Museum, University of Rochester. Price, $0.80. 



