VARIETIES OF OYSTERS. 139 



each with its receiving and distributing chambers, or 

 auricle and ventricle as they are respectively called. 

 (CasselVs Popular Educator.) 



The Order of the Lamellibranchiata is divided into 

 two unequal groups, distinguished from one another by 

 the number of the adductor muscles. 



( i ). The Mononiyaria, having one muscle, placed nearly 

 in the middle, or rather towards the back. 



(2). The Dimvaria, having a separate muscle placed 

 right and left side of the body. The impressions or scars 

 made by these muscles, on the inside of the shell, serve to 

 instruct the geologist as to which group every bivalve 

 belongs. Some of the members (e.g., certain species of 

 Lucina, Tellina and Thracia,) are said to have only one 

 branchial leaflet or gill on either side the body, but in all 

 probability this leaflet is double, although united and 

 apparently single. 



The British Monomyarian Mollusca comprise the 

 families Anomiidae, Ostreidae, and Pectinidae ; all the 

 remaining families are Dimyarian which is a very large 

 group. The late Mr. Clarke says that the only true 

 Monomvarian Mollusca are Ph. o las and Teredo : other 



j 



conchologists do not agree with him. Dr. Jeffries does 

 not attach much importance to the form or presence of the 

 pallial scar, being the mark left on the inner margin of the 

 shell by that part of the mantle which adheres to it, and 

 keeps the rest of the body in its proper place ; although 

 this character mav serve to recognise certain genera. 



O o 



In some families the mantle is open on all sides but 

 the back for the admission of food and water, as well as 

 for the ejection of faecal matter ; while in others it is 



i 



