322 CONCHIFERA. 



the hinge-margin, as in Iridina and Crenatula ; or it is 

 small and placed in a cavity between the primary teeth, as 

 in Venus and Cardium. The hinge is placed immediately 

 beneath the beaks, and is usually composed of one or more 

 primary or cardinal teeth, and a lateral or secondary tooth, 

 situated at a little distance on each side. Sometimes the 

 hinge- teeth are numerous and uniform, and arranged along 

 a straight hinge-line, as in Area and Nucula ; sometimes 

 lateral teeth only are developed, as in Lasea and Alas- 

 modon ; and occasionally they are altogether wanting, as in 

 Mytilus and Serripes. 



The adductor muscles, which serve to close the valves in 

 the Conchifera, are two in number in the Veneridce, Car- 

 diidte, and some other families, but in the Ostreida and 

 Pectinidce there is only one, whence these Mollusks have 

 been arranged by M. Lamarck and others into Mono- 

 myaries and Dimyaries, but in some genera, as in JEtheria 

 and Mulleria, there are two adductor muscles in the young 

 state, and but a single one in the adult ; while in Tridacna 

 they are central and blended together. The impressions 

 these muscles leave on the interior of the valves is some- 

 times double, and occasionally there is a third scar pro- 

 duced by the muscle of the foot, as may be seen in the 

 Pectens, while in some few genera, as Anomia, there is a 

 fourth caused by the muscle of the plug, and sometimes 

 there is found a small scar under the beak, formed by the 

 retractors of the foot. 



The muscular mantle-margin also leaves an impression 

 on the inside of the valves which is called the pallial line, 

 and which is sinuated behind when the animal is possessed 

 of retractile siphons, but which is entire and simple when 

 no siphons are present. 



