536 Mollusks. 



after deposition. Sometimes, however, they all advance together, and 

 then it is remarkable to see how closely they are compacted. This is 

 frequently so firmly done, that the young escape in very distorted 

 forms; in some the lateral rims of the aperture are pressed closely to- 

 gether ; in others the beak is thrust backwards upon the shell, but is 

 more commonly twisted laterally : these and many other distortions 

 are frequently to be seen. 



When the shell has escaped from the " cup " it is perfect in all its 

 parts and general form, except that it is destitute of the spiral cone ; 

 this is acquired afterwards, and enlarges with the growth of the ani- 

 mal, without being embraced by the mantle. R. Q. Couch. 



Polperro, February, 1843. 



Microscopical Society of London. 



March 20, 1844. — J. S. Bowerbanlc, Esq., in the chair. 



Read, a paper by Edwin Quekett, Esq., " On the structure of the Ligament unit- 

 ing the Valves of Conchiferous Mollusks." After some preliminary observations on 

 the nature of univalve shells, in which he considered the operculum as a step towards 

 the second shell of bivalves, he stated that the usual opinion was that while the strong 

 adductor muscles inserted into the inner part of the valves served to keep the shells 

 closed, the ligament attached to the hinge performed the office of opening them by its 

 elasticity. But upon examining shells of different genera, such variations in the po- 

 sition of the ligament were found as to render this solution of the mode in which it 

 performs its office, in many instances incorrect. Thus in the oyster and cockle the li- 

 gament is situated without the hinge, while in the muscle and scallop it is within. 

 Now it is evideut that to produce the same effect, a power behind the fulcrum must 

 operate in a contrary manner to one before it. The former can only do it by expan- 

 sion, after compression produced in shells by the closing of the adductor muscles ; 

 while the latter can only effect the same end after having been elongated by the same 

 means. This contrariety of action induced him to examine the structure of this liga- 

 ment in various shells, and he found, in addition to those differences which may rea- 

 dily be observed without the assistance of the microscope, that while, in many cases, 

 no perceptible structure can be perceived in the ligament placed before the hinge, in 

 the common muscle {Mytilus edulis) it appears under a high magnifying power to be 

 composed of a dense tissue without any particular structure, in which appear certain 

 small channels or lacunae filled with fluid. Hence it would appear that when the li- 

 gament is compressed by the adductor muscles closing the valves, the fluid in these 

 lacunae being incompressible, renders the ligamentous structure more tense, and there- 

 by increases its elasticity. The external ligament, again, has long been known to be 

 composed of two layers of substance possessing different organization, as stated by Dr. 

 Roget in his ' Bridgwater Treatise,' (i. 217). Upon examiuing these as they exist in 

 the oyster, cockle, &c, by the aid of the microscope, the external layer exhibits no 

 marks of structure, whilst the internal one is seen to be composed of numerous fibres, 

 each about ^ of an inch in diameter, running parallel to each other, and apparently 





