566 ANOMIIDiE. 



Sub-gen. monia, Gray (?Tedinia, Gray). 



Shell with the surface of the valves racliately ribbed. Per- 

 foration of lower valve large, only slightly embracing the large 

 thin plug. 



Alope, Gray. ruacrochisma, Desk. 



Cepio, Gray. patelliformis, Linn. 



claviculata, Cpr. pernoides, Gray. 



Colon, Gray. Zealandica, Gray. 

 Tone, Gray. 



Genus PLACENTA, Retzius. 



Shell semi-transparent, flat, sub -orbicular. Ridges of 

 the hinge very gradually diverging from each other, the 

 hinder ridge much the longer. Muscular scar rather in 

 front of the middle of the hinge. 



Ex. P. orbicularis, Retzius, pi. 129, fig. 3. Shell, P. 

 orbicularis, fig. 3, a, 3, b. 



The structure of the shell in this genus has been ascer- 

 tained by Dr. Carpenter to consist of sub-nacreous, plicated, 

 separable lainellse, which are occasionally permeated by very 

 minute tubuli. The single species described is from the 

 coasts of India and China. 



Genus PLACUNA, Solander. 



Shell purplish, sub-opaque. Hinge-ridges rapidly diverg- 

 ing from one another at about the angle of forty-five 

 degrees, of nearly equal length. Muscular scar under the 

 centre of the hinge. 



