SHELL OF FRESH-WATER MUSSEL. 125 



reous lamellae which enter into its composition. The calcareous lamellae 

 alternate with organic layers. 



An elastic ligament unites the two valves along the hinge-line. It 

 serves to open the shell and is antagonised by the two adductor muscles. 

 See next preparation. 



The inner surface of each valve is marked by three principal muscular 

 impressions, as they are termed, two anterior and one posterior, placed 

 somewhat dorsally. Of the two anterior impressions, the larger is due to 

 the attachment of the anterior adductor muscle, as well as of the anterior 

 retractor of the foot ; while the smaller, placed nearer the free margin of 

 the shell and more dorsally, corresponds to the protractor of the foot. The 

 larger portion of the posterior impression gives attachment to the posterior 

 adductor muscle, but its irregular process, extending dorsally towards the 

 hinge-line, denotes the point of attachment of the posterior and larger 

 retractor of the foot. A continuous line extends between the impressions 

 of the two adductors parallel to the margin of the shell and at some little 

 distance from it. This is the pallial line. It gives attachment to a series 

 of muscular filaments which extend outwards into the free edge of the 

 mantle, and are attached to the spot where the epicuticula commences. The 

 epicuticula being flexible, contraction of these muscles brings the edges of 

 the epicuticular layers of both valves into firm contact, and at the same 

 time retracts somewhat the free edges of the mantle lobes. As the pallial 

 line describes an even curve throughout its whole extent, Anodonta is said 

 to be integripalliate. 



Muscles pass up from the foot and are attached to the ridge which 

 borders the ligament internally, and in shells with hinge-teeth (infra) to 

 the teeth, which are merely developments of the ridge present in Anodonta. 

 A muscle also passes across from the ridge of one to the ridge of the other 

 valve. And small muscular bundles are attached also to the inner part of 

 the ligament. 



The valves of the shell are inequivalve in the Ostreidae, one valve being 

 smaller than the other. Each valve is nearly equilateral in some of the Pectens, 

 e.g. Pectunculus. It is a rare thing for the anterior portion of the valve to be larger 

 than the posterior. Anodonta, like some of its immediate congeners and some of 

 the oldest Lamellibranchiata, geologically speaking, is devoid of ' hinge-teeth ' on 

 the inner aspect of the valves. The hinge-teeth consist of ' cardinal-teeth ' placed 

 below the umbones : of ' anterior lateral teeth ' in front of the ' cardinal ' teeth and 

 ' posterior lateral ' behind them, and below the ligament. Unto, which belongs 

 to the same family as Anodonta, and, like it, inhabits the freshwaters, possesses 

 anterior and posterior lateral but no cardinal-teeth. The presence of these inter- 

 locking teeth gives a dissimilar appearance to the two valves when viewed from 

 within. The variations in the character and arrangement of the hinge-teeth have 

 furnished recently a basis for the classification of Lamellibranchiata (Neumayr). 



