 The shell is added to in two ways ; the valves grow larger when more material is secreted by the mantle at the margin of the shell , and the valves themselves thicken gradually throughout the animal 's life as more calcareous matter is secreted by the mantle lobes . Although the ( sometimes faint ) concentric rings on the exterior of a valve are commonly described as " growth rings " or " growth lines " , a more accurate method for determining the age of a shell is by cutting a cross section through it and examining the incremental growth bands . Use of this technique has changed views on the longevity of many bivalves . For example , the soft @-@ shell clam ( Mya arenaria ) was thought to be short @-@ lived , but has now been shown to have a lifespan of at least 28 years . 
