To relate the rings on the surface of the shell of M. trossulus to the growth lines that . are formed annually in the nacreous layer of the shell Lutz, 1976 , we made radial sections of the valves of 90 Mytilus collected in our study areas on Montague Island and Knight Island in 1996. As broad a size range of mussels as possible was selected haphazardly from samples of mussels collected at four to six sites chosen at systematic intervals along the shore within each study area. The mussel shells were prepared for . aging using a method modified after Rhoads and Pannella 1970 . All tissue was removed from the mussel shell, and the valve with the least erosion was selected for aging. The valve was washed with detergent to remove any lipid residue, dipped briefly in a 1.0% HCl–water solution, rinsed immediately with distilled water, and dried. Each Ž. valve was imbedded in a block of epoxy resin Epofix; Struers, Westlake, OH . A radial Ž. section of the imbedded valve was made using a precision cut-off machine Accutom-2 with a diamond blade. The section was made perpendicular to the surface of the valve Ž along a plane that bisected the umbo and the posterior edge of the valve the vector of . maximum growth . The cut valve surface was wet-polished sequentially with 3600, 6000 and 12,000 grit polishing paper on a grinding wheel and then buffed with 0.05 mm Alumina buffing powder on a felt buffing cloth on the wheel. The polished surface was etched with a 1% HCl–water solution for 105 s, then immediately blotted dry with paper wipers. The etched shell surface was flooded with acetone and a piece of sheet acetate 0.5 mm thick was applied to the etched surface. The acetate peel was removed from the shell surface after 6.0 min and examined for growth lines under a compound microscope with phase contrast optics.