What is "achalinus"

Achalinus (common name: odd-scaled snakes) is a genus of harmless snakes in the family Xenodermatidae. They are found in Japan, Taiwan, China, and northern Vietnam. Nine species are currently recognized. Achalinus was previously placed in Colubridae (along with other xenodermatids). (If anyone knows why exactly they are "odd"-scaled, please add that information to this section.)