384 CONCHIFERA. 



of dress, not unfrequently met with in the cabinets of concho- 

 logists. 



(418.) Taking a more general view of the Conchiferous Mollusca 

 than we have hitherto done, we shall now proceed to consider the me- 

 chanism for opening and closing the valves of the shell in which 

 they reside ; an operation effected in a very simple and elegant 

 manner. 



The shells are connected posteriorly by means of a hinge 

 differently constructed in different species. In the Oyster we have 

 an instance of the most simple kind of junction. In these Mol- 

 lusca a mass of elastic ligament, composed of perpendicular and 

 parallel fibres, is interposed between the posterior edges of the 

 shell, and so disposed, that by closing the shell the ligamentous 

 mass is forcibly compressed while at the same time its resiliancy 

 is such, that, immediately the compressing power is withdrawn, it 

 expands, and thus forms a simple spring calculated to keep the 

 valves apart, and cause their separation to a greater or less extent. 



The antagonist to this elastic force is the adductor muscle 

 (jig. 176, c), a fleshy mass of very great strength, the fibres of 

 which pass directly from one valve to the opposite. The adductor 

 muscle, although in this case single, consists of two portions of dif- 

 ferent texture (Jig. 177, /, m) ; so that it would appear to be formed 

 by two muscles closely approximated, so as to compose a single power- 

 ful mass adapted to keep the valves in contact with a force propor- 

 tioned to its massive size. All those species having a single muscular 

 mass, such as the Oyster and Pecten, have been grouped together 

 by conchologists under the general name MONOMYARIA, while 

 another and more numerous division DIMYARIA, is characterized by 

 having two adductor muscles distinct and widely removed from 

 each other. The Mussel tribe and many others are examples of 

 this arrangement which is represented in subsequent figures. 



Simple as the structure of the hinge is in the Ostracea, in other 

 Bivalves it frequently exhibits far greater complexity, and the op- 

 posed valves present prominent elevations and deep fossse which 

 lock into each other, and thus form a very secure articulation of 

 great strength and solidity. In such cases the arrangement of the 

 elastic ligament for opening the valves is slightly modified, being 

 placed externally instead of within the shell, but its action in 

 antagonizing the adductor muscles is still equally efficacious. 



(419.) We must, in the next place, solicit the attention of the 

 reader to a very important subject connected with the economy of this 



