grJTe^s 



ITH the Oyster and its allies 

 we reach the third order of 

 bivalve mollusca — the 

 Pseudolamellibranchiata — a 

 small order characterised by 

 a further advance in gill - structure. The united 

 filaments of the gill are not merely so long as to 

 require folding back until they practically constitute 

 four gill-plates on either side of the foot, but the 

 edges of these plates may be united to the foot and 

 the mantle respectively. The edges of tlie mantle 

 are quite open all the way round, the foot is very 

 small, and the foremost of the two muscles for closing 

 the valves is usually undeveloped. The only native 

 representatives of this order are the Fan Mussel 

 (Pinna), the Oyster (Ostrea), the Scallops (Pecten), 

 and the File Shells (Lima). 



The Fan Mussel (Pinna Orudis) has the distinction 

 of being our largest native mollusk, adult specimens 



75 



