certain Genera of Conchifera. 



25 



stinct ; generative organs combined with the right mantle-lobe. 

 Ventricle exposed, not perforated by the rectum. 



There is an admirable memoir on the structm-e of Anomia by 

 Dr. Lacaze-Duthiers, in the Ann. Sc. Nat. 1854, t. ii. p. 1, with 

 figures. These and some drawings by Mr. Albany Hancock 

 have been compared with the example here represented. 



[This description differs in many particulars from that given 

 by Mr. Clark in the ' British Marine Mollusca.' — J. E. G.] 



Placuna placenta, Singapore. 



Animal nearly symmetrical, free (or attached by a byssus when 

 very young). Mantle quite open, its margin fringed with large 

 and small cirri, and furnished with an inner pendent border. 

 Adductor round, subcentral. Gills as in Anomia, their outer 

 margins grooved. Pallial muscle reduced to two fasciculi in 



Right valve with the animal, as seen on removing the left mantle-lobe. 

 /, ligament ; c, cartilage ; p, pedal muscle ; /, foot ; h, liver ; g, gene- 

 rative gland ; v, ventricle ; a, adductor ; i, rectum ; h, 6, branchio- 

 pallial muscles. 



front of, and one behind, the adductor. Pedal muscle minute, 

 anterior, attached to the upper (left) valve. Foot small, cylin- 

 drical, tubular (very extensile ?) . Lips short and wide, becoming 

 striated inside near the gills. Generative organ and rectum 

 attached to the right mantle-lobe. Ventricle exposed, not per- 

 forated. 



It will be seen by the figures that Placuna is essentially like 

 Anomia ; both are very dififerent from Ostrca, and more like the 



