FISHERY BULLETIN: VOL. 82, NO. 1 



10mm 



5mm 



FIGURE 1.— (a) Right valve of an 1 8-yr-old ocean quahog,Arctica islandica, 71.4 mm in shell length recovered 

 on 21 August 1979. Estimated annual growth was 1.3 mm. The notch-mark area is shown before (h) and after 

 (c) peeling off the periostracum. (d) Optical photomicrograph (microprojector) of seven repetitive growth 

 lines in an acetate peel of the valve margin. An arrow points to an interruption of growth and growth line 

 formed at or soon after marking the clam in 1978. An internal line is_evident between the mark-induced line 

 and the valve edge. This line is the normally occurring age mark probably formed in late autumn-early winter. 

 Scale bars of magnification are included. 



FIGURE 2. — (a) Left valve of an ocean quahog, Arctica islandica, 

 about 110 yr old and 100.5 mm in shell length recovered on 20 

 August 1979. Estimated annual growth in 1 yr was 0.1 mm. The 

 notch-mark area is shown before (b) and after (c) peeling off the 

 periostracum. (d) Optical photomicrograph (compound micro- 

 scope) of many repetitive growth lines in an acetate peel of the valve 

 margin. An arrow points to an interruption of growth and growth line 

 formed at or soon after marking the clam. Scale bars of magnification 

 are included. 



