CONCHIFERA. 27 



Class CONCHIFERA. 



Order LAMELLIBEANCHIATA. 



The principal characters of this Order have been al- 

 ready given in the former volume. A few of the mem- 

 bers (e. g. certain species of Lucina, Tellina, and Thra- 

 cia) are said to have only one branchial leaflet or gill on 

 each side of the body ; but in all probability this leaflet 

 is double, although united and apparently single. The 

 Order is divisible into two unequal groups, which may 

 be distinguished from each other by the number of ad- 

 ductor muscles. The first and smaller group (MoNO- 

 MYARIA) has but one muscle, which is placed nearly in 

 the middle, or rather towards the back. The other and 

 far larger group (DIMYARIA) has two separate muscles, 

 which are placed on the right and left sides of the 

 body. The scars or impressions made by these muscles 

 on the inside of the shell serve to instruct the geologist 

 to which group every bivalve belongs. The British 

 Monomyaria comprise the families Anomiida, Ostreidce, 

 and Peciinidce. All the remaining families are Di- 

 myarian. The late Mr. Clark says that the only true 

 Monomyarian Mollusca are Pholas and Teredo ; but his 

 observations in this respect do not agree with those of 

 other conchologists. I do not attach much importance 

 to the form, or even the 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 may serve to recognize certain genera. In 

 some families the mantle is open on all sides but the 



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