54 



NORTH AMERICAN FAUNA 68 



Figure 28. — Cross section of the right upper canine tooth of an adult female 

 sea otter, weight 47 lb. (21.3 kg.) taken at Amchitka Island, 12 June 1956. 

 Although various zones and lines are revealed by refracted light in this 

 thin section, no series or rhythm of growth layers relating to the annual 

 cycle could be defined. No technique is yet developed to reveal chronological 

 age from a study of the teeth. (National Bureau of Standards photo 

 45-56) 



