same means to keep out intruders from our habitations. This oper- 

 culum is capable of being adjusted so as to be suitable for its pur- 

 poses. 



If we can imagine that this moveable operculum could be jointed 

 by a hinge or some such contrivance, to the edge of such an univalve 

 shell, we should then see that this condition of structure would be 

 analogous to a rudimentary valve, though it would be very unlike the 

 other, still the transition from this arrangement to a shell composed 

 of two more or less equal valves can be very easily reconciled. 



The creatures inhabiting a shell composed of two more or less 

 equal valves are denominated by zoologists " conchiferous mollusks," 

 and the shell is termed a bivalve, in contradistinction to that inhabited 

 by the other division of molluscous animals of which we have just 

 spoken, which is denominated an univalve. 



With this compound mansion it is requisite that there should be 

 provided an apparatus by which these valves may be opened for the 

 purposes of the creature within, and closed against the enemies with- 

 out. It has been said that in those which occupy a single valve, 

 the opening and closing of the orifice of the shell is effected by 

 the protrusion arid retraction of the creature's body bearing an oper- 

 culum. 



In the conchiferous division these methods do not obtain, and it is 

 found that an apparatus exists which is compound in its nature, and 

 not a portion of the animal as far as the great purposes of nutrition 

 and reproduction are concerned, by which these actions are secured 

 to these creatures. 



One of these organs is muscular, its action depending on the will of 

 the animal. The other is mechanical, that is, the action is effected 

 by a power something analogous to elasticity, which is not destroyed 

 even by death, but continues for a considerable period to possess this 

 property. 



In all conchiferous animals the closing of the shell is effected by 

 one or two powerful muscles extending from one valve to the other at 

 some convenient spots, depressions, or want of nacre or polish, gene- 

 rally indicating the attachment. 



The contraction is exceedingly powerful, and the creature is capa- 

 ble of maintaining it for a lengthened period. The position of the 

 muscle for effecting the closing of the shell, is placed generally not 

 far from the centre, and maintains throughout the growth of the shell 

 a relative distance between the hinge and the margin of either valve. 

 On the inner side of the muscle are generally some ligamentous fibres, 



