Island line on thumb tag at Destiny Palmistry.
Every so often hand analysts consider the island line on the thumb. However, it is not practised much in modern palmistry. I have included it here because it is something that interests me, and I watched these lines on my thumb change over the past twenty-one years.
The crease at the base of the thumb is called a family ring, and some believe that these lines can depict events regarding family. The line in the middle of the thumb is meant to show much the person cares about their family or how much involvement the person has in their family. It apparently depends on the depth and strength of the line.
The crease in the middle of the thumb (top phalange) is also about the health of the individual. It usually has an island formation, much the size of a grain of rice. If present in the perfect form, it is said to show a healthy, happy and robust family life. If broken, split or missing, the family life might be going through change or may have had disruptions.
Read the thumb lines article here